We want to get to know people a little bit before they pay/register. We're not interested in having vendors come that won't add value to the gathering, for example. And we don't want any one school to take too many of the seats. The conference attendance is capped, and so although it's generally first-come-first-served, we just want to make sure attendance is also balanced.
(And if we're already full for this DesignCON, we'll put you at the front of the line for next time.)