"Maintenance".
For one, as you might be able to tell, SRB2.org's server is already having a somewhat rough time. I've been a little frustrated by it but there's little to nothing I can do about it, after all.
For two, the MS is pretty poorly coded -- much more so than the game itself -- and heavily reliant on things that a master server really shouldn't rely on. Like, for instance, this specific version of vBulletin. It's not even fully complete, and has been falling apart for a while.