Because no one played those anymore. I can honestly say I've not entered any of those since Season 3. They always gave crap for rewards... "You ranked #1! Enjoy your green mk X useless console." "And everyone else, enjoy your free Aux Battery!"
And the Big Dig one took forever to do unless you had a crack team.
There were some instances where it took 2hrs to finish, assuming everyone didn't just bail.
So I guess we all get to grind more (but different) old content cause that's where the Dil went.