Because you generally want your Set IOs to be at the max level possible for that set, in order to provide the max possible bonus%.
Also, I have a number of alts, OCD, and have been known for builds to die in their 20s and 30s. The question isn't "Now that I'm in my 20s/30s, what sets am I going to slot," but "Will I care enough about this build to play it after hitting level 50?" I just don't want to invest into any sets that low. Common IOs are less to think about and I have characters who have all of the crafting badges so making them is cheap and easy. Far easier to have a bin sitting there with numerous level 25 common IOs versus low level IO sets. "Wait, I don't have a melee set? (shakes fist)."