Probably because the contest threads are moved to the contest forum that no one looks at and therefore it dies a slow and agonizing death! 

Back when they allowed contests to be posted in the forum that they were for, you saw more people posting contest announcements and more people entering them.

I found out first hand that it's now against the rules for people to even posts links in the Poser forum letting people know of existing contests. A while ago I saw a contest in the contest forum that looked fun. I posted a link here pointing people to that thread and encouraging them to get involved. The thread was deleted and I was told to not do that anymore.  Yet not long after that I saw a different mod post a link in the freebie forum pointing people to a  contest  that was posted in one of the modeling forums (not the contest forum).     I can't see how that was any different than what I did, yet it was against the rules for me, but not for them.

So now when I see a contest on another site, I can't let anyone know about it other than in PM, which I think is a real shame.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi