Basically, Due to the nature of how the challenges are set up and how long thay had been running the decision was made to end them.

They are meant to be inclusive, and while thats generally a good thing, it means your much more limited in what you can put up as a challenge. for example, I could think of 100 challenges around the new robotics content. BUT. that would be placing a paywall barrier to entry on the challenge, as only those with the DLC could participate.

Participation was also down.
I blame that in part on the fact that it was a weekly challenge. The concept of a regularly scheduled challenge is a good one, It was rather the frequency that i fell did it no favors. 1 week sounds like lots of time, Till you have a 40 hour full time job and kids. (or many other previous commitments) and a lot of people, myself included, lost motivation to complete a challenge once the week ended.