Same reason people say brand X is better than brand Y... personal preference. Simple as that.
I've used Kenwood and Pioneer coaxials off HU power in the past and I wasn't disappointed with them. Throw an amp on them and, well, it is a different story. The only Pioneer speakers I would ever amp and expect very decent sound reporoduction are the REVs and the 720PRS's.
2010 Kia Forte SX
2009 Kia Borrego EX 4x4
Setting Amp Gains:
Bought from: jacobcopeland87, 95bat, alxmlr789, na_rsx