Why won't my car speakers get loud?

i have a 1998 blazer and the door speakers arnt loud at all. i have a aftermarket radio. its a jesen phase liner flip out, but the reason why i ask this is, i never seen anybody with subs have to amp up there door speakers but me? my subs are loud but the boor speakers are quiet, even when the subs are off, you have to turn the radio up all the way almost just to jam out to the music! i have the door speakers amped up with a 2 channel 1000w amp, and it sounds great i was just woundering why isnt it loud without the amp?