Son of the Serpent
Pupil
The first half of what you said is actually something i already pointed out and supports my points. (Edit: woopsie. I have dyslexia for the record. Totally thought you said "are" where you actually said "arent". Considering that, i was wrong about agreeing with the first half. Also its not in the rules that cantrips are shorter casts but seems to be a thing. Not definitively. Just probably.)Cantrips aren't faster than leveled spells of the same casting time- they're simpler. They are spells that a caster has practiced so often and so repeatedly that their second nature to them, and thus require no resource expenditure or real effort to implement.
It's not stated anywhere, but it immediately jumps out to me that the idea behind the bonus action casting rule is that casting a spell that quickly temporarily damages your ability to cast magic over a certain complexity threshold- a threshold that cantrips and cantrips alone reside below (at least thus far).
THE SECOND HALF on the other hand...thats certainly not canon or in the rules, but if i didnt hate theae rules so much id very much like to use that as an in game explanation for why the restriction is there. I like that a lot. And it makes an awful lot of sense (unlike the rules). Might even use that for some things in my 3.5 games.
I love it when someone comes up with good fluff to go with the rules.