There's one major reason: The user experience is horrible.
Instead of presenting a simple "username/password" box, you present an OpenID box. The user enters their OpenID, which looks like a web address (e.g. http://claimid.com/yourname). They're then redirected to the OpenID provider, log in there, have to choose whether they trust realmtech, then are redirected back.
Suddenly a one-step process has turned into a three-step process, with a redirection away from the site (in this case, realmtech).
At the moment, the most promising solution to our identity crisis is Windows Cardspace. It solves the problem by basing authentication on the desktop, instead of via a redirect, yielding to a far better user experience than OpenID. But given the history of Microsoft-invented identity technologies, this might go the way of the Passport.