In all honesty, I imagine it's just that it's not a story Ridley wants to tell so someone else is. ADF is the kinda guy that if he feels something needs doing, he'll give it a go.
I wouldn't be surprised if this prequel was a result of ADF reading the script for Covvie and going "we need to tell the story of Shaw and David" or some such. That's me assuming about what the prequel is about.