Mostly because Disney and modern guests seem to value a hard number of attractions they can accomplish in a day. Not the amount of time physically spent in a ride vehicle. Otherwise Energy would still be a thing.
I think what you're describing, however, can be attributed to changes in how much of a park guests can conceivably experience in a day. In the past, except on the busiest of days, you could go to the MK, spend a day, and experience pretty much any attraction or show you wanted, some of them twice. People didn't mind longer attractions like the Country Bears, Tiki Room, etc. You could relax and just enjoy them and still experience everything you wanted.
That isn't the case any longer. Now, with the massive increase in visitors, FastPass+, dining reservations, and leap in the cost of admission, guests who want to experience as much of the park as they can and feel like they've gotten a fair return on their substantial financial investment must either spend multiple days or plan their single day down to the minute. Even then, it really is impossible to see everything at the MK in a single day (most of the time) and experiencing favorite attractions multiple times means that something else has to go.
It's a shame, because I'm one of those who is all for a longer attraction that is done well. I didn't mind either of Energy's incarnations because I felt like I was given a quality experience from beginning to end (though both incarnations had their faults). Splash Mountain is one of my all-time favorites, mostly because of it's length. It's one of those rides that is so enjoyable that, whereas a lesser attraction would end right after the big drop, Disney delights us with another show room with a rousing finale. To me, it's a perfect attraction.
I think, honestly, that longer attractions could make a comeback, but to do so the root causes of the very real phenomenon you described need to be addressed. I think that, eventually, they will be, somewhat, once guest satisfaction dips below a certain level and attendance numbers drop. The possibility of the latter happening anytime soon is next to none, however, at least until after the 50th. It will be interesting to see what happens once that is out of the way and Galaxy's Edge isn't the "new thing" any longer. That's several years off, though, which means that, if you're a fan of the longer shows like the Tiki Room, etc. you'd better enjoy them now. Park real estate is more valuable than ever and for every new thing that they want to put in, something else is likely to close. Adventureland is so congested now, with the silly Dumbo-alike Magic Carpets, that the large Tiki Room building feels like it's barely holding on.
Sorry for being so long-winded. Just finished my second cup of black coffee this morning.