Pure diminishing returns. Why spend resources beyond when the number of users gets rather small?
Heck, I’m an LTS guy and waiting for 18.04… and I feel like the dinosaur minority with all the hype on 16.10 which is supported for a WHOLE 9 MONTHS! 
By the way, after 3 years the Ubuntu stuff will continue coasting for 2 more years and the chances of something breaking are small to need support for the Mate stuff. The repositories will last even beyond that. I just checked… my Ubuntu Server 10.04 (Apr 2010) still has active repositories!