Makes sense narratively to me. In the in-game world, a round is 6 seconds, so a single character turn is possibly only a second, sometimes less. It takes a bit of time to cast a spell, often it involves some complicated gestures, along with a vocal component, sometimes incorporating material components. It'd be hard to justify casting 2 "complicated" spells in a second or less. A cantrip is often just a flick of the wrist so it makes sense for one major spell and one cantrip. The caster waves an arcane sigil into the air, speaks a few magical words and fires out a major spell, then follows up with a quick snap of the fingers for a cantrip. That makes perfect sense to me. Going through the motions of 2 "full" spells makes less sense to me in terms of how time works according to the rules and common sense.