Once a PC is put in play, giving them rewards IC for actions OOC is detrimental to continuity, having to explain how this is coming about IC, divesting additional DM resources to an 'elite group' of PCs, as Mayhem says.
And how do we avoid the 'bare minimum' trap that Mayhem speaks of?
There is also simply the fear of someone else 'getting something' that you can't get. Course, IMO, that's simply how life works. You do something, you get something. Work for reward.
FI wrote up this thing for 'rewarding' contibutors... don't know where she posted it, but it looked fairly reasonable to me

Maybe if people ask her, she'll post it for folks to see.
Ultimately, you are going to have people who are dead set against this sort of thing regardless of how reasonable it is, their minds are closed. Identify who those people are and stop addressing their posts.
That actually goes for a lot of issues. You'll never convince them, stop trying.