I think the reason they did this was to make all the classes play a little more different from each other. In ME1 and 2, the primary difference between the classes concerning playstyle was what weapons you were trained with. Combat stats were what determined the playstyle of the tech/biotic characters. This is glaringly obvious if you play an adept in ME1 and pick up assault rifles as your new game + power. The class plays like a squishy soldier with some CC.
By making the difference be the amount of weapons you can carry vs the types of weapons you can carry, it forces the tech/biotic powers to stand on their own to make their respective classes feel unique, and changing the cooldowns is an essential part of that.