Why did the ratings for Game of Thrones drop for the last season(s)?

Question

I’m watching Game of Thrones for the first time. So far, I’m partway through the third season and am enjoying it.

However, I remember hearing that a lot of the fans were disappointed with the way the last season ended for some reason.*

Here is a plot that I’ve made of the (scaled) average ratings per season with data from Rotten Tomatoes, Metacritic, and IMDb:

This shows that the last season was poorly received, but the Metacritic reviews suggest that problems were starting to show since Season 6.

What was it about the last season(s) that made the ratings drop so severely?

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* No spoilers please.

Answer 1

No source material and the lack of George R.R. Martin's influence starting from Season 5

Game of Thrones followed the books closely until Season 4, but then the showrunners had to make up stuff as the next books were not finished yet. To add to that, George R.R. Martin (author of the A Song of Ice and Fire books) claimed to be "out of the loop" by season 5. Martin's involvement, from script consultation and writing to casting decisions to set visits, had been a cornerstone of the show's quality in the earlier seasons. 

With no more source material and the lack of Martin's influence, the writing faltered.

As Condal got to work on “House of the Dragon,” he leaned on Martin’s expertise a lot — the opposite of what had happened with Martin in the later seasons of “Game of Thrones.” In the early seasons, Martin wrote and read scripts, consulted on casting decisions and visited sets. Over time, however, as he stepped back to focus on his long-delayed next “Thrones” novel, “The Winds of Winter,” Martin grew estranged from the show, which was created by D.B. Weiss and David Benioff.

“By Season 5 and 6, and certainly 7 and 8, I was pretty much out of the loop,” Martin said.

When asked why, he said, “I don’t know — you have to ask Dan and David.” (A representative for Weiss and Benioff declined to comment.)

- "Can ‘House of the Dragon’ Be HBO’s Next ‘Game of Thrones’?" The New York Times

Rushed storytelling in the final two seasons

A Forbes article aptly titled "It Is Now Clear Having Two Short ‘Game Of Thrones’ Final Seasons Was A Mistake" pinpoints a critical factor in the ratings' decline: rushed storytelling. In a bid to conclude the series, showrunners David Benioff and D.B. Weiss opted for a mere 7 and 6 episodes for the final two seasons, in contrast to the usual 10.

Teleporting character arcs and plotlines

The showrunners cramming the story into fewer episodes meant complex character development got sidelined. Characters experienced drastic shifts that felt unearned due to the lack of proper build-up. With less time, characters suddenly appeared across vast distances, creating a sense of disorientation.

Martin also envisioned the story continuing beyond eight seasons, with longer seasons to flesh out the plot. The showrunners deviated from this, impacting the overall quality. HBO also reportedly wanted more episodes and was willing to provide additional resources. However, the showrunners prioritized their desire to move on to other projects.

Martin disagreed with Benioff and Weiss’ decision to cut the show short. The author long envisioned “Game of Thrones” to run for 10 seasons. Martin’s agent, Paul Haas, revealed last year that the author would fly to New York City to meet with then-HBO CEO Richard Plepler in order to convince him to extend the series’ television run.

“George would fly to New York to have lunch with Plepler, to beg him to do 10 seasons of 10 episodes because there was enough material for it and to tell him it would be a more satisfying and more entertaining experience,” Haas said. “[Weiss and Benioff] were tired, rightfully so. They were done, and wanted to move on, so they cut it short and then negotiations became, how many seasons can we stretch this out? Because of course HBO wanted more.”

Martin published a blog post before “Game of Thrones” aired its final season in which he wrote how “complex and a little sad” it was to be ending the show in its eighth year. The author added, “I wish we had a few more seasons. But I understand… I’m sure some of the actors were signed up for like seven or eight years, and they would like to go on and take other roles. All of that is fair. I’m not angry or anything like that, but there’s a little wistfulness in me.”

- George R.R. Martin Kept ‘Out of the Loop’ on Four ‘Game of Thrones’ Seasons and Doesn’t Know Why: ‘Ask’ the Showrunners. Variety

This confluence of factors—a lack of source material, George R.R. Martin being "out of the loop", rushed storytelling, and sidelined character and plot development—ultimately led to Game of Thrones' conclusion and the decline in ratings.

Answer 2

Right off the bat, I want to point out that Game Of Thrones had such a wide audience that there are myriad different opinions on how good its ending was and what it should have done. My answer is but one of many possible interpretations. However, given that you're asking for a spoiler-free answer anyway, it seems that I can navigate away from the murkier subjective waters of which parts of the story could've been resolved better, instead sticking to the general consensus in most online discussions on the topic, which I believe to be sufficiently objective for the purpose of answering the question.

TL;DR The resolution of the story wasn't bad, but it was told in a hasty manner. There was a significant drop in writing quality and attention to detail, when compared to earlier seasons.

Imagine if I want to write the final book in my book series. I'm going to wrap up the stories. After writing, it turns out that it's a 3,000 page novel. But my publisher limited me to 300 pages, no more.

Rather than rewrite my story or condense its narrative, I decide to simply rip 2,700 pages out of my book and release the remaining 300 pages. Readers are missing a lot of subcontext, and the story seemingly jumps from one plot point to another without giving you any breather inbetween or building up any narrative.

This is, at its very core, what went wrong in the last season. Too much content to wrap up, not enough screen time to walk you through it, thus rushing you past the needed exposition and foreshadowing. The lack of exposition and foreshadowing in turn made the conclusion feel like a lampshaded and forced twist, rather than the natural ending to the arc.

I won't discuss any specific arcs because of spoilers for obvious reasons.

In the end, I think the completions of most arcs would have been much better received if there had been more storybuilding to lead up to its conclusion. I don't think the arc resolutions were bad, but it required the viewer to make major leaps to understand how the characters would go from [end of season 7] to [end of season 8].

The distance between these two positions was big (for some arcs - not for others), and there simply wasn't enough screen time to actually walk you from point A to point B. Based on the opinions I read online, the negative focus was almost exclusively on arcs that were very different from what we expected them to be (at the end of season 7).

You, or others, might disagree on a per-character basis whose arc did or didn't make sense, or felt rushed, or did not end in a satisfying manner. But that's very subjective and not meaningfully answerable.