Because “been there done that”. See A "who-does-what-where-and-has-what-say" primer
I won’t fault anybody else for trying to make an alternative to MDN but if they do they shouldn’t be surprised to get a lot of “don’t count me in” responses—especially from people working on browser-engine projects or who’ve otherwise been involved over the last 10 years or whatever with work on developing the core set of Web-exposed interoperable technologies that we all want documented and explained (as MDN attempts to).
@stuartpb all the problems with MDN that you’ve outlined would seem to be fixable with changes to MDN. In my experience, the people leading the work on MDN are highly responsive to feedback and suggestions for refinements. So I’d personally suggest that instead of trying to get an alternative off the ground, the energy would be better spent getting involved with MDN and helping to make it better.