I imagine cantrips were buffed because, as a whole, spells and spell-casting were heavily nerfed relative to prior editions. Or, at least, prior editions with the exception of 4th edition. Spells and spell-casting were made weaker with the reduction of spell slots with few ways of boosting their number, the addition of the concentration mechanic, and allowing re-saves for almost all controlling effects. Certainly their were were buffs to some spells within the context of the system, but as a whole, spell-casting got the nerf bat relative to 3rd edition.
Why consider 5e's spell system relative to 3rd edition instead of 4th? Largely because of the mass exodus of players upon 4th edition's release, I imagine. I don't believe 4th edition was the base for 5e playtest material, yes there were some terminology carried over, but by and large little of 4th edition was carried over into 5th edition.
