Well, mainly because Dan Larimer is building EOS :) So he probably believes in it's success!
So he would never build on platforms of his competition like NEO or ETH or NEM!
Dan Larimer built both Bitshares and STEEM, and EOS is his current project. Once he finishes that, apparently he will build a Steemit competitor on top of that.
O wow I wasn't aware of that. That makes sense! :D I'm not gonna worry too much about it yet. Healthy competition is alwyas nice and will keep the developers of Steemit from slacking off. But it is something to keep an eye on in the future when things might get a bit more specific :).