I'm a subscriber of the slower update cycle explanation as well. Everyone I know who owns a tablet is happy with what it does for a longer period of time. Have a look at iPhone refreshes versus iPad refreshes and you'll notice that the iPad follows the iPhone, so it should be no surprise where the iPad falls in the growth trajectory of the company.
Following that same line of examination (looking at refreshes), consider the impact of each refresh. What does a new iPad include?
Thinner and lighter? That's a nice to have, but is it cost-of-upgrade nicer?
Touch ID? How important is Touch ID to users who use the iPad casually. With an iPhone you're frequently operating the device with one hand, so Touch ID simplifies the authentication process. When you're using your device for payments, Touch ID is pretty damned incredible. Neither of these apply to the iPad. I'd love to have Touch ID on my iPad, but it's not worth the cost of an upgrade.
Better camera? I'm an outlier here — almost everyone I know uses their iPad as a camera, except me — but I'm just not sure this alone is enough to push people to upgrade. In the iPhone environment, all of these new features act cumulatively to make an iPhone upgrade worthwhile.
Larger size? Oh, wait. That's iPhone only, and it's probably one of the biggest drivers of iPhone 6/6+ sales. It's irrelevant to the iPad.
Tablets, as a product, involve a slower upgrade cycle because they're actually the more traditional product when compared to a device like the iPhone.