The carriers bear some blame as well. Apple negotiated for the right to upgrade the OS without carrier review, but most Android device OEMs don't. So the carriers drag their feet as well, hoping you'll buy that new device from them and be locked in for another 2 years.
This is it; unless crucial apps (or some other necessity) only work with a newer Android version (which could push OEM's to update) they never will. Much better for OEM's to hope that you'll "update Android" through the purchase of a new phone.