Ipad is a victim of its own success.
There is really no pressing need to upgrade if you have an iPad 2 or later because it is still good enough.
It is also more of a consumption device meaning as long as it can handle content, users wont feel a need to upgrade.
Lastly it is also not a phone, meaning the standard upgrade cycle of 1 to 2 years don't apply. It is more in line with a computer which is about 3 years.