It's not really a problem, and there are a few popular ideas that get done on yearly basis.
But I'd guess that the general idea is to keep the challenges fresh and, well, challenging, which is why you don't see the same prompts returning that often.
So yeah, it would not be the first time a previously done idea gets done again, and I certainly agree that it can be a great thing. There are a few older challenges that I've absolutely adored and would love to see again - maybe I'll at some point get to run some of them myself? I haven't really gotten into the challenge running part of this CL business yet.