The critical difference, and the reason we are bothering to try, is that it's easier to frequent multiple websites than it is to adhere to multiple standards.

Yeah, but it's still difficult. I post at the same name on the xkcd forums (that's me at number 7), except basically every post I've made there in the last few years (after discovering Less Wrong) has been to discuss My Little Pony or video games, and both of those are only because those are mostly off-topic here.

Valuable participation on a website takes effort- and so if you're just looking for people to read your blog, then you can expect that to about the degree that you write interesting posts. But if you're looking for people to build a community with you, then any effort those people direct towards you comes at the cost of effort they could have directed elsewhere. For example, a new organization with a different focus was spun off the organization behind LessWrong- and posts by the people in the daughter organization, several of whom used to be core LW members, are few and far between, and mostly advertising for their organization. (While I miss their posts, I think that they're doing more important work over at CFAR, so I'm not complaining.)