There was an article about this in The Athletic recently. The FA sells the TV packages for the FA Cup as standalone contracts and has successfully managed to have two UK broadcasters involved for years now. The League Cup is sold by the EFL as part of a big package that also includes league games and the playoffs. It wants an exclusive broadcast partner and Sky has been the only interested party for some time. Two games per round is what they’re happy with - viewing figures don’t justify any more than that.