How about the reason is because someone who just bought a mini at the beginning of March might be totally pissed if a retina mini comes out a month later?
I think some practical information might be in order here.
The Apple Stores specifically seem to have quite a bit of leeway on an individual basis when dealing with returns/exchanges.
To your point, I suspect that if you bought a mini, and the mini with retina came out a month later, they would probably exchange it for you. The probably part of this would be based on how you treat them.
In other words, if you are nice about it and ask and instead of demand and have not returned six other items to the store already they will likely take care of you.
Personal anecdote to that point.
I bought a macbook pro about 2 years ago that died about 6 weeks after I bought it. I took it in expecting just to get it fixed. As I was doing that I asked a couple of questions about how to get along while I waited for the repair. The tech paused, said "hold on for a few minutes", walked in the back room and about 5 minutes later came back with a brand new unit.
Can't say that will always happen, but it did for me and is one of the things that keeps me buying Apple