Makes sense. I'm pretty sure they felt forced to do that to avoid front-loading classes when it came to multiclassing. Of course, that still happens, and they found a way to avoid the worst of it when they came up with multiclassing anyway. On top of that, they created feats that allow enough "little dips" into other classes to make multiclassing less important, so they could have gone with level one subclasses anyway.
In 4e, the initial classes where uniform in advancement. At level X, you gained feature type Y. The feature you actually got was different based on the class you where in, but the type of feature was the same.
5e actively moved away from this. Uniform subclass progression would have been easy, but by discarding it it makes the game less like 4e. And part of the goal of 5e was to address every complaint anyone ever had about 4e and say "look we solved it".
Martial classes can do more than swing a sword? FIXED. The 5e fighter just swings a sword. An optional subclass can do some tricks.
Fighters are complex? FIXED. The champion subclass of the fighter is ridiculously simple.
Wizards can't do everything? FIXED! Wizards in 5e have a wide set of game-breaking spells. Mitigated with the concentration mechanic, spell DC control, and spells that offer more than one chance to break free.
Non-Spellcasters can heal by shouting? FIXED! In the base game, only spells can regain HP (except during rests).
Bard, as a magical class, can do a small amount of shout-healing during short rests.
Ranger isn't a 3e ranger, but some kind of awesome skirmisher? FIXED! Just like the 3e ranger, the 5e ranger is a mess.
Stacking small modifiers is dominant yet annoying? FIXED! Advantage replaces a bunch of conditional-but-you-can-guarantee-it mechanics.
Weapon attacks roll more than one weapon damage die? FIXED! Except crits, there aren't any 2[W] damage die (or higher) abilities in 5e.
High-level characters have too much AC? FIXED! AC doesn't scale in 5e.
High-level characters are awesome without focusing on things or magic? FIXED! 1/2 level bonus removed, proficiency bonus only applies to things you are proficient in.
Now, this is a bit of a joke. I like both 4e and 5e. But you can see the direct "we take your complaints seriously" parts of 5e design.