There are other reasons as stated, such as the keyboard and trackpad.
I think of the question in reverse: why would people want to pay more money for a Kaby Lake? Granted, it feels pretty silly to shell out $2K for a 3 year old machine. But it also feels silly (to me, anyway) to pay more for something that is a step backward in multiple ways.
Of course, everyone is different, and there are different opinions. Some people care more about performance. Some people care about the new displays (which are clearly better), or the newer speakers (although if you're listening to music on your laptop speakers...I ask you...WHY GOD WHY?).
But I can tell you that I'm a heck of a lot more comfortable on my 2014 15" rMBP than I am on my 2017 15" rMBP. And that's why I'm typing this on the former, and the latter is sitting in the other room on a shelf.