Shorter sequences save time for audiences, and money for producers
There is an excellent article on the history of tv show credits here: A brief history of TV shows' opening credit sequences
In it, it talks about how after a tradition of longer sequences, the need for them declined as audiences wanted to get straight to the action.
Following the example of Lost, and its 15 second intro, other shows began adopting a similar approach to shorter sequences:
[Lost’s] minimalist approach demonstrated TV shows didn't need credits or a catchy theme songs to set a tone and get viewers; cast members could be introduced during the first scene, and people would still watch. Title cards also allowed shows to "dive right into the action," providing more screen time per episode. And of course, minimalist credits meant money saved.
A few years later, and it’s clear that audiences responded to the change:
By 2006, only about 10 percent of shows used a theme song or credit sequence to set up the story. "Clearly, brevity is key," The Associated Press reported in 2006. "No drawn-out intro or hokey theme. Networks don't have time for that — and neither, prevailing TV thinking goes, do the country's couch potatoes."
My own view is that with binge watching more and more common, the need for an engaged, long-term, viewer of a show to sit through a long introduction is reduced. They already know the characters, the setting and the theme - they’d rather get to the story.
This is evident by Netflix’s introduction of a ‘Skip intro’ button.
