It looks like the British show Sherlock gets a new season every 2 years (2010, 2012, 2014, 2016?) based on IMDb. Why so much time in between? They only release 3 episodes 1.5 hours long each. With every new season it feels like you need to start watching from the very beginning to remember details of this show as it is so important.
It's because of scheduling.
When you have two leads who are also featuring in major films like The Hobbit... (together!) and Star Trek plus having writers and producers who are involved in a variety of projects, it's difficult to get everyone in the same place at the same time to make a show... particularly if you want to maintain the quality of the show at the level people are accustomed to.
From an article in The Telegraph:
[Moffat] confirmed he hoped to make further stories for the popular BBC ONE show but said it was a case of fitting in with everyone's diaries.
"Well, apparently Benedict and Martin are quite popular in the movies these days so it's quite difficult to schedule around them. And obviously Mark and I have our other commitments too, but it's just a matter of scheduling. We're all keen to continue."
Here's another article from Collider in January of 2014 that explains it in even more detail... but the basic info is the same.
Even though Sherlock hasn’t quite finished its Season 3 run on Masterpiece on PBS, fans of the popular series are already anxious to know when it will return with new episodes. Thanks to the increasing work schedules for show stars Benedict Cumberbatch and Martin Freeman, who have very busy acting careers, and Steven Moffat and Mark Gatiss, who write the episodes, the gap between seasons doesn’t seem as though it will shorten, anytime soon.
Are you working extra hard to shorten the gap between seasons, or do you just have to deal with scheduling?
MOFFAT: We deal with scheduling. I’ve also got to do Doctor Who. I’ve got no choice about that. That’s the day job. Everyone is a little bit busy. But also, it’s worked to make them wait. If we made Sherlock the ordinary way, and did a run of 6 or 12, it would have been over by now. It would have been done because Martin and Benedict would never have been able to find the time, after the first [season]. It would be done. This model of the TV series will happen again. Because we all love it so much, this could go on for a very, very long time. You’ll get to see an awful lot more of it.