Brazilian Jiu-Jitsu fighters don't compete in Judo competitions because they do not have the necessary skills, throwing, sweeping, and a diverse array of takedowns. The Brazilian Jiu-Jitsu fighter may be superior on the ground, though not necessarily because many Judo schools practice Kosen Judo, both gi- and no-gi, and remember that Judo has just as many joint locks as BJJ, though focusing mainly on the arms, and even a greater number of faster, more efficient chokes and strangles, as Jiu-Jitsu was derived from Kosen/ Kodokan Judo. And lest you forget, Judo is a form of wrestling, and as such, includes various wrestling and pinning techniques and reversals that BJJ does not.
So even though the Brazilian Jiu-Jitsu fighter might be able to submit the Judoka, the Judoka only needs a single clean throw to win the fight. And modern Judo competition rules do limit the time allowed on the ground, putting the reactive and not proactive Jiu-Jitsu style at a fatal disadvantage. And as it is improbable for a Judoka to be out-thrown by a Brazilian Jiu-Jitsu fighter, Judo players remain dominant in their form of competition.
And don't forget the outcome of the first documented Brazilian Jiu-Jitsu match versus Judo. The founder of BJJ, Helio Gracie, fought Masahiko Kimura, often called "the greatest Judoka of all time."
Kimura won when Gracie's corner threw in the towel after Helio had been thrown via uchi mata, etc., several times, and then had his arm broken via bent armlock, aka a keylock, aka Ude Garami, and later named the Kimura lock.