When in doubt, assume Ezreal is the reason for restrictions on items. Memes aside, if the mana restore was just on-hit, Ezreal probably would have single handedly kept the mana restore so undertuned as to be worthless. (Effectively lowering the costs on his already cheap Q would make his poke even stronger.)
In fact, the item did restore mana based on damage at one point. Before the pre-season marksman update in 5.22, the item didn't have any crit at all (it had lifesteal instead). The mana gain was an on-hit effect that, if I recall correctly, was 2-8% (based on missing mana) of damage dealt. During the marksman update, this was shifted to have crit (as it is one of the "stat-bindings" for marksmen). The crit requirement is probably both to require attacking and further crit item building to be effective.
The %max mana vs flat mana lets it be better if you do build mana without making it too strong if you don't. The %max mana vs % damage dealt makes it more consistent instead of swinging up and down depending on how many items you have and what targets you are hitting. Ironically, the need to crit does make the mana restore somewhat less consistent, but given that you will end up with 50% crit to makes the CDR, it shouldn't be too swingy because of need to crit.