It's the whole back to the dungeon thing.
It makes a certain sense if you think of 5e as trying to emulate the basic old explore as far into the dungeon as you can style of play. (Even then it has issues, it's slower than something like B/X, making it a lot harder to get through all those fights in an evening of play, while the easier fights lack the tension of earlier editions where one or two bad rolls could still see the end of your character).
It's also I think a product of how much PCs get. They now have twice as much effective hit points a day, due to hit dice, while casters now have effectively more spells due to cantrips as well. And you couple that with trying to build all that onto what is effectively 3e's maths with the expectation that there is still a sort of parity between how pcs and npcs are built, at least in that they use the same spells and do the same kind of weapon damage and you basically need more encounters.
For whatever reason, perhaps simply because the playtests revealed that people liked them, they weren't willing to strip out 4e add ons such as at-will attack magic and the ability to self-heal. (Although the latter also makes encounters a lot easier to effectively balance).