F1 has managed for many years to get at least 10 teams on the grid. In the past they had often way more candidates than available places.
Meanwhile wrc struggles to keep 2 teams.

The difference? F1 does not require an official car maker to build a car. F1 does not require expensive homologation procedures that can only be done by some big marque.

Anyone who can find the money should be able to build a wrc car. I'm sure a lot of compagnies could build very capable cars if they would be allowed to.