It's individuals responding to the incentives before them. If something goes wrong because they're using IPv6, it's their fault. If you never upgrade anything until you're forced to, then you can never break anything by upgrading, and you're never seen as a "stuff breaker".
This is it. Having broken stuff by upgrading myself, I can say it does not make you popular. This is especially the case if what the upgrade did was tighten up some bug that it turns hid a bug in your team's code...