It's just their way to ease people into the game instead of instantly obliterating the servers. To help them monitor how the game handles a populated environment slowly. (Although, right now, it seems to be bugging out because of the high population at the moment.)
Also, why not? I've known about the development of this since November 2003. We've been patient enough.
Another thing is that not everyone from those years are going to instantly hop on once they are given access. I'm from 2003 (and I was also chosen for the initial beta testing, but I had no time) and I started playing when the 2005 users were let in since I've been busy IRL and hadn't realized the release until then.
It's just a matter of patience, really.