I can't imagine any managers at Twitter were sitting there thinking "Hey, I have this great idea that will make us boatloads of money! Let's release an open-source html/css/js framework that makes it super easy for anyone to make a site that looks like Twitter."
In all likelihood, it was a project that the developers wrote for internal ops and later attempted (and succeeded) to open source. If you look at the Impact Graph on github (https://github.com/twitter/bootstrap/graphs/impact), you can see that it's pretty much Jacob and Mark's project.
From Twitter's perspective, they can either let Jacob and Mark continue to run with it at no real cost to themselves (since it's open source and they can still use Bootstrap), or they can try to find someone within the organization to continue to maintain it, which has a very real cost -- developer time. Given that they may not find someone to fill that role and that forcing someone into it would probably be bad for the project, I think the decision makes a lot of sense for Twitter.