The iPad 1 does not have contract lock-in and is not actively being sold by Apple. Apple is not facing serious tablet competition right now so what incentive do they have to support the iPad 1? What are people going to do, throw out the iPad 1 for a Kindle Fire? Maybe if Windows 8 or Android get their act together, but right now, Apple is chasing demand and has little incentive to support old hardware when they can push you into the new one. This is why the tablet space needs some real competition.
Bingo. Apple still sells new 3GS phones, so it would directly impact customers that may be buying their products today. This isn't the case with the iPad1.
Apple rarely does things (solely) based on technical requirements or some architectural vision. It may be frustrating, but it is consistent - they are primarily customer focused, not specs-focused or vision-focused. Yes, there is attention to specs and an underlying vision of computing, but they are 2nd to the (paying) customers.