Why do some seller on ebay not leave feedback?

i am waiting for feedback on 5 items three from the same seller,
the items have arrived my feedback has been left,numerous emails have been sent asking politely if when they are next on ebay could they please leave me some.I know they are on ebay because they have other items on sale.
Why doesnt ebay make a rule that everyone has to leave feedback it does my nut when people wont leave feedback.(the oldest item was back in feb and still has not left me any)