Do you mean the music itself, or the title sequence?
Well, they are still there, if shorter.
I think with the increased advertising and trailers for other programmes, American shows are getting shorter running times, so I dare say the title sequence is kept short to allow for more story.
A one-hour show in the 1960s would last for 50 minutes without adverts, now a one-hour show often lasts less than 44 minutes, without adverts.