There is no official answer on this specifically that I'm aware of, however it's likely a part of WotC's attempts to cut down on the Wizard's versatility.
In earlier editions, especially 3.5, the Wizard had access to just about any kind of spell you could think of. This played a large part in making the "Batman Wizard" who was able to deal with just about any situation, given just a bit of time to prepare. In 4th edition, this was scaled back sharply, too far for some people's tastes, and 5e seems to be working to compromise between those two extremes.
As for why Enhance Ability specifically, it's even harder to say for sure, but I'd guess that WotC decided that, since buffing is strongly associated with Divine spellcasters like Clerics and Druids, it would make for a good place to start cutting into what the Wizard can do. Wizards still have many buffs, of course, but most tend to work more directly, either granting the target something to use (Magic Weapon, Magic Armor), or altering them in more direct, often superficial ways (stoneskin, invisibility). Clerics and druids, on the other hand, tend to buff in a more holistic way, changing or enhancing the basic abilities of the target, affecting them from the inside out, rather than the outside in, which follows the theme of healing and caring for people's souls.