Just speculation on my part, but most likely to see what kind of market was out there and for production reasons. Where as now, they know what the market is already, so no need to set up preorders.
First off, I don't know that there's a "probably" around preorders for or against; I'd tell people not to *expect* them, but I'd stop short of predicting if they will or won't. Be prepared for either, we'll find out whenever they're announced.
With the ipad1, they had no existing device to cannibalize sales of by allowing preorders. Of course they do iphone preorders, so who knows.
I'm sure you'll see a lot of ideas, but unless someone's plugged into the high level decision makers I don't think there's a way to know the real reasons. One could make arguments for why to do it either way; enhance excitement, force greater lines for more publicity, etc. In theory, selling ipads in-store would take less time than selling phones (no need to integrate with the carriers, upgrade issues, etc) so less incentive to direct a higher portion of early sales to online vs in-store. Tough to say why one way or the other with any certainty IMHO.
I understand that it's unknown at this point whether or not pre-orders will be allowed this time around, just curious why people think they won't and why Apple didn't allow them with the iPad 2.
Like I said, it's just curiosity getting the best of me, nothing more