Because they're probably not undergoing a formal QA process yet and Apple doesn't want to deal with bug reports and bad press about how its beta is janky/buggy/sucky in other browsers. This is pretty standard stuff for webapps doing a phased rollout of browsers -- it's not anything unique to Apple.

Just because people in this thread have found workarounds to get it to launch in Firefox doesn't mean there aren't still a bunch of bugs there, that Apple is aware of but doesn't want marring the user experience yet.