That depends on their particular reveneu/investment/'charity' model. I think currently some of them are, to a certain degree, competing.

Theoretically, although I don't want to imply that this is necessarily the case, Steemit inc (whom I suppose controll the Steem.io site) could of course deny listing any apps that they think could be hurting their bottom line. At this point in time however , I have a hard time seeing why they would.