Apple's selling a lot of iPads currently:
That's a crapton of iPads. I think the lineup is potentially confusing to customers, and I think there are a lot of factors as to why Apple's done this.
I think it's become clear that the iPad is not an upgrade-every-year class of device, but without going to a 5/5S-style naming and upgrade scheme, Apple's sending mixed messages about what it thinks customers want.
Likewise, I think it's last year's deal of having the iPad mini and iPad Air being the same will probably prove to be an anomaly. Maybe Apple's confused about what it itself should be doing with these products.
There's one big factor at play here besides the upgrade cycle, however: since the introduction of the iPad mini two years ago, the average selling price of the iPad line has fallen dramatically.
I think the old devices in the lineup, coupled with the lack of meaningful updates to the iPad mini are all about solving this problem. If someone wants the best iPad, the more expensive, bigger one is the clear winner.
Apple's at odds with itself, though. On one end, it's trying to push people up to the iPad Air 2, and on the other, it's trying to compete with low cost Android tablets. This plan leaves nerdy consumers in the middle; people who would happily have upgraded to a new iPad mini now won't, making the update cycle problem worse.
While the iPad Air 2 looks like a great upgrade from the iPad 3 or 4, upgrading is not tempting, speaking as iPad Air owner. With TouchID being the only feature separating the iPad mini 2 and iPad mini 3, I don't see why any iPad mini 2 owners would upgrade this year, either.
I think this year's iPad lineup is being used by Apple as an experiment. The company is trying to figure out how the iPad mini should stack up. It's an unusual thing to see from Apple, and I think a lot of customers would prefer a simpler product line with more clear reasons to move to a newer device.