Because KF21 isn't a mature product yet (only air superiority capability now). And French with their Autonomie stratégique make us can use Rafale more "freely" than US stuff (or stuff with a lot of US tech).
Rafale can carry more fuel too, hence more range.
Now F15EX got its own PSP and knowing SM pushing hard for US fighters purchase to secure GSP, I think save to say Eagle and Viper are pretty much secure now.
I hope US gov and Boeing okay with Ngeteng scheme 😂