We have a general thread on this which is stickied, but as a result can be 'hidden in plain sight'. Which means that making a topical thread about this would be relevant when there seems to be a spate of challenges that fall apart (because none of the guidelines in that lengthy post are being followed).
I actually agree, I've been also looking at all the challenges that have been going on lately. I'm going to actually be a dick and hold forth on my opinion on the named challenges I've seen. No offence to those hosts, I'm purely looking at the challenges:
The slump is at least in part due to the changes in balancing between betas and ongoing bug ironing (which is why the V6 Supercars Series rebooted, and still has plenty of good potential). SoMWoM is a good well thought out challenge too, but is dependent on the ongoing development of BROBOT (though we're also waiting for the sports car reviews!).
As for the rest-
Pro Touring Tour had a nice idea but having the rules satisfy both the agenda and users is tricky (already discussed this enough) and above all else, shifting the playing field after submissions begins is too troublesome. It's dead for now but like Kristina said, I hope it can be properly revived.
Formula One Role Play is extremely ambitious in scope and while it's still building in momentum, I still find a lot of the logistical details very vague. The vaguer and more ambitious the idea, the more worried I get that it'll turn into vapor because there's no plan on how to tackle the major things (like the tracks needing to be rebuilt when BROBOT develops further).
Group B Rally: I think "you better not pull a rileybanks" was a pertinent comment
but more importantly, we still need clarification on what the hell the "offroad recommendations" mean. Are you going to make it a rule, or what? Because as far as we can tell, "recommendation" doesn't mean squat when the proposed track and premise doesn't make sense anyway.
2005 Best In Class: entertaining premise, all the data is collected as far as I can tell. We're counting on your to eventually pull through lordred!
1993 DTM: as leo said, highly restrictive materials and component rules combined with a minimum weight is a round about way of restricting the body (why not then just stipulate a minimum wheelbase?) That'd be the main turnoff.
J.E.C.B: come on, get the rest of the results released!
This leaves a total of 3 tournaments that were inexplicably abandoned in progress (more if any of the above named are also declared dead), which, all things considered, is lower than I expected.