Taking the analogy to a house, Arc is like a half finished construction project. The frame is up, they're still putting insulation in, and there's one finished room with a cot, a table, and a hot plate which Paul Graham is using to cook the news.yc software. Sure, you might attract scads of interior decorators by opening your house up to random strangers, but very few programmers will be willing to come over to live in Arc. They'll stay in their own houses which are already well stocked with comfortable furniture, decorations, etc., even if it is all a little cluttered and you have to go through the kitchen in order to reach the library from the bedroom.
Some of the people speaking out against Arc are genuine trolls, but some are just saying in a non-tactful way that they'd prefer to stay in a house with drywall, thank you very much.