I don't necessarily believe it's because the Reserves/Guard are run differently. At the end of the day, we all belong to Mother Blue, so the entry requirements are going to be the same.
However, my thoughts are that the AD recruiter doesn't want to deal with someone who is a possible in, when he can deal with someone who is a definite in, in the eyes of the AIr Force. Why go through all the trouble of recruiting you, and working for you, if in the end, he/she won't get any payoff from their efforts if your waiver is denied?
Shitty deal, but with today's economy, there are plenty of people to choose from that don't require a waiver.
You could always enlist and then apply to the OTS boards as soon as Tech School is complete. That would alleviate any potential "DQ's" for you, as you would have already had them documented at MEPS, and it wasn't an issue (unless it's something that would preclude you from passing an FCI at Wright-Patt).