They don't have the same buying cycles because they are aimed at different markets. The phone is aimed at being the all things to all people device. Everyone has one and you take it with you and use it for almost everything when you are on the go. Phone calls, browsing apps email GPS etc and every innovation (LTE, NFC etc means you can do these things better) therefore the pressure is on too keep this device up to date.
iPads are aimed at a different market, most iPad owners already have a phone of some sort so the thing about an iPad is that you use it when the phone doesn't quite hit the mark because you want a bigger screen. For many tasks the portability and ubiquity of the iPhone means that this is the first choice.
As a result web browsing and playing games are probably the biggest reasons that people use an iPad.
I read somewhere that a large portion of iPad usage is people sitting in front of the TV web browsing etc while they are watching the TV. I suspect watching content on a plane, train or bus is also a major use case. If that is what you want it for there is not so much pressure to upgrade it as often. If you only use it on the couch or in bed using WiFi, then LTE and NFC are irrelevant. If you only look at the device in the commercials the pressure on having the fastest performance etc is just not there. Therefore an iPad will be good enough for a long time.
I bought the first iPad when it came out and waited almost 4 years before upgrading. In the same time I had upgraded my iPhone twice. I love my iPad and would not part with it but I will hang on to it much longer that a phone. That does not make it a bad device - just different