I don't think it is the rating system since you can just rate both in the same tournament we do that all the time in Denmark.
I think the two reason is
- You have to pay a fee to FIDE when doing FIDE tournaments. And that makes the tournament more expensive.
- I think (and someone correct my if I am wrong) they is some different chess rules from USCF <-> FIDE moving pieces with two hands when castling is allows in USCF I think but not FIDE and also some draw position rule are different if I remember correct.
Those are correct. It costs significantly more to rate a tournament under FIDE. Also, you have to have a licenced arbiter, which takes additional things that a lot of tournament directors either don't want to mess with or don't have the directing experience to qualify for.
Note that I am talking about pre-Covid. Obviously, it is very difficult to hold tournaments during a pandemic.
In the United States, there are many USCF tournaments but few FIDE tournaments. Why is this? If it is because the U.S. has a USCF rating system, why not scrap that rating system and replace it with FIDE ratings? Otherwise, how do you get enough FIDE-rated players to attend your tournament?