Earlier quoted context omitted.
I think that Google saw that artificial scarcity worked well with gmail and thought the same would be true for Wave. The problem, of course is that a Gmail account didn't depend on anyone else having one to be useful. Wave, in theory was an open standard as well, but in practice, Google was and is the only provider.
I think more a more likely explanation is that they lacked the infrastructure to roll it out effectively. If that's the case then they should have just waited until they could deliver. It's pure conjecture of course!
I got a very early invitation, and it was pretty slow, and got worse as they added more users. They fixed that of course, but it did slow down the roll-out.