Life expectancy & diminishing returns...
are the major factors here, surely? People expect a tablet to last a while, so the replacement timescales are closer to a laptop than a phone, and when the improvements have become as incrementally small as they have the incentive to upgrade isn't there anyway.
The processor may be "twice as fast" as the previous model but if, in the real world, this isn't actually noticeable then why bother?
Oh, and this isn't just true of Apple gear - this is the first year I haven't seen any need to update either of my Nexus devices. Both the 5 and 10 continue to perform very well, and having put Lollipop on the 5 it's almost like having a new phone anyway. From a hardware perspective at least, it definitely feels like we've got to the point of rapidly diminishing returns.