You asked "why"..

Because buyers are what makes Ebay money. Not sellers.  Ebay has stated that their research shows that negative feedback puts buyers off returning and buying again.

If theres a million sellers and 100 buyers, not good. If theres 100 sellers and a million buyers, great. Ebay can more than afford to lose sellers... not so much buyers. Their undoing will be assuming sellers and buyers are different, static, groups.

Im sure more sellers wouldnt care about not being able to leave negative feedback, given the strike system now, if Ebay treated sellers a little better, or in the least, respected their rights and acted fairly towards them.