Another difference is that many people use the iPad as a secondary machine. They have their phone with them at all times. So if the iPad is a little sluggish at times, you won't be too bothered. But if your phone is being slow, you're gonna want to replace it right away. I think that factor, combined with the upgrade cycle, resulted in many people buying an iPad, and except for the really hardcore Apple fans, have just kept that same iPad up until today. Again this is all based on speculation, but from what I have seen from average iPad owners (not techies in the SF bubble) they are happy with their iPad 2/3 purchases even now.

Also don't forget to consider that when the time to upgrade an iPad comes up, you can decide not to upgrade it and wait until it's completely dead or whatever. With the iPhone, you can usually sell your 2 year old phone for at least $200 and use that for a free upgrade to the next model. Since most people are gonna keep their plans for that time frame anyway, it's stupid not to use the subsidy.