I have been searching the last couple of weeks for a reason why Sorcerers and Bards have access to Enhance Ability while the Wizard does not.
Is there a clear cut reason behind this or is it a mistake that it wasn't added and wasn't caught during Q&A tests?
Has there been any official words on this (and my google-fu has been particular bad)?
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.
Agent Paper is correct in the fact there is no official answer.
The present array of ability enhancing spells had their immediate origin in Dungeon & Dragon 3.0 with the spells Bull's Strength, Cat's Grace, etc. A range of 2nd level spells for enhancing a character's abilities temporarily. And the Wizard possessed all of them along with a selected few by other classes.
The d20 spells had their origin in the Greyhawk Supplement to the original 1974 edition of Dungeons & Dragons. Strength was a 2nd level magic-user spell.
Strength: This spell increases a fighter's strength by from 2-8 points (roll dice after spell is cast). It will also increase a cleric's strength by from 1-6 points and a thief's by from 1-4. When a fighter's strength reaches 18 or higher due to this spell an additional determination of strength is to be made as already specified for strength of 18. Duration: 8 game hours
This was continued through the various editions where it was expanded in later edition to encompass enhancement to all the attributes.
With D&D 4e focus on tactical combat, the abilities of each class was balanced in accordance to their role. The wizards role did not include buffing so the strength spell was dropped.
D&D 5e restores many but not all of the historical abilities of the various classes. It looks like the Strength spell in the form of Enhance Ability was not one of the historical abilities restored. Whether this was intentional or an oversight it is up to the Wizard's Team to say.
You of course have the option of restoring the historical spell lists. If you need a reference I recommend using the OSRIC retro clone available here. Or the Moldavy/Cook B/X PDFs found here and here.
