I'm not even sure the dollar price was the issue. But I would have to rebuild things like "people to follow", get used to the new interface, have the risk that the platform doesn't survive - a lot of implicit costs. What could be working is to offer a paid tier for an already existing service like Twitter - but unfortunately they have a strong incentive not to. Because it would mean that their ads would stop reaching the very people that the advertisers are interested in - the ones who are willing to spend money.