The closure of "Web Design" was not an issue of gaining critical mass (that's typically done in Area 51); it was an issue of scope.
I copied over the closure notice here:
I'm Pops, one of the Community Managers at Stack Exchange. Our team spent some time talking about the Web Design beta today, and we have decided to shut it down effective Friday, 3 October 2014.
Two months ago, when this site was a proposal progressing through commitment in Area 51, I outlined some of the team's concerns in the Discussion Zone. Our primary worry was that this site would be based almost exclusively on topics from existing sites, causing "splintering" within the network for those topics without producing any benefit in return. Unfortunately, that seems to have happened. The questions on this site are almost entirely questions that could have been asked elsewhere on the network, where they would have gotten comparable answers. For example:
I picked these four posts by looking at a random list of questions on the site. (To be fair, I did skip some, but most of the ones I skipped could have gone to SO; I just didn't want to have a list that was a dozen items long.)
Now, covering topics that are accepted elsewhere doesn't necessarily make for a bad site — if Web Design clearly met the needs of a distinct group that weren't being well-served anywhere else, we would take that into account. But that's not the case here. Indeed, the hallmark of this community thus far has been an inability to agree on the most fundamental issue: what this site is about. It's not for lack of trying; two thirds of the posts on meta have something to do with scope or topicality. But several of those posts reached no solid conclusions, and most of the ones that did come to conclusions resulted in topics being discouraged or declared out of scope rather than allowed or on-topic.
Not much was going on on the main site while — and perhaps because — the scope issues were being worked on on meta. As I write this, a mere 57 questions are up on the main site, which is pretty low for a week in beta. Whatever the reason, the fact is that the activity just isn't here.
Ultimately, we're not seeing a reason why this site needs to exist as a separate entity from Stack Overflow, User Experience, Graphic Design and other sites already in the network. They already have communities of experts ready to address nearly all of the questions that are being asked here.
As mentioned, the site will remain open until Friday, primarily so that anyone wishing to preserve the excellent questions that did get asked can request their migration to one of the other sites on the network, or archive them by other means.