> He is able to condense so much complexity into his works, and he speaks to the heart as equally as he speaks to the intellect.
Just to add to that-- the complexity of Bach is something like going half-way around the circle of fifths in the middle of a long fugue in G#-minor. And he does this not just for kicks, but because this is one in a 24-part polemic to push other composers/musicians to use his favored equal temperament tuning system. "Using my system, you too can visit foreign keys with confidence and ease! Never sound out of tune again!" That's the whole point of Book II of his Well-Tempered Clavier.[1]
Similarly, Mozart's complexity was taking a social issue-- like egalitarianism-- and sneaking it into an opera by quickly composing 3 dances of different classes (and meters!) to be performed concurrently on the stage. Apparently he cued the on-stage musicians for each dance when he conducted the premiere of Don Giovanni.
I mention the latter because Bach's favored textures were thick and busy, and Mozart's tended toward light and effervescent. There's a tendency to confuse texture with deeper musical complexity, and that can lead people to overlook Mozart's contributions and/or give Bach credit for the wrong things.
1: Lazy theory-- Bach wrote Book I so the keyboardist could tune first using equal temperament, then choose any key and sound in tune. But most collections of pieces (e.g., dance suites) were all in the same key anyway, so this wasn't much of a practical advantage. However, if he modulated to various keys in a single piece, then those keys would sound poor in just intonation. Then the musician would be forced to use equal temperament to play the piece! Unfortunately, not all of the fugues in Book II are as harmonically adventuresome as the G#-minor fugue, so a lazy theory it remains.