Gosh I wish I could claim the scoop but it seems Robert Scoble found out the answer when he asked the Gmail Project Manager why the project has not been rolled out to everyone. Seems they do not have enough servers and only as they add servers do they allow existing users to hand-out more invites.
“First session I attended? I met Chris Uhlik of Google. He’s the program manager of Gmail and the Google Toolbar and a few other things. He gave a talk at 11 p.m. to well after midnight. I asked him why they only give out a few Gmail accounts at a time. Remember, I thought it was sheer marketing genius. Turns out it wasn’t. Turns out they don’t have enough server capacity to deal with everyone who wants an account. So, when they buy a new server and get it installed they hand out another round of invites. Heh, their server salesperson must cheer everytime he or she sees more invites being advertised on weblogs.” [Scobleizer]