Because it means when the Mini comes out, everything that's ever come out for iPad will be completely compatible with it from the start. It's a logical move for them... plus it means when they upgrade it next time round 1st gen buyers will have more of a reason to upgrade.
You don't seriously believe that has any bearing on matters when you consider the second part of your post and also the fact they're happily sending other new products like the iPhone 5 down a new aspect and resolution route? They've made it perfectly clear that 'backwards compatibility' is of no concern to them by changing their propriety connector and obsoleting swathes of add-ons and accessories.
What would have been sensible was a 7 inch version of the iPhone 5 display.