The very top Judokas? A few simple reasons.
-In love with the Olympic dream. It's more common to dream of winning a medal than fighting in MMA.
-Outside of the US, the Judo power house nations (Japan, France) compensate their top Judoka EXTREMELY well with phantom jobs, stipends, corporate sponsorships, etc. Outside of Rousey and the top draws in the sport, you would make more as a top Judoka in Judo than in MMA whether you're a man or woman.
-Inside the Judo world, fighting in MMA is viewed by many as not being any better than underground street fighting. In other words, it's viewed by many as both a massive step down in prestige and disrespectful to Judo's core traditions.
-In France it is actually considered ILLEGAL for a Judoka to teach MMA, compete in MMA, or even compete in tournaments for other martial arts. It is grounds to be banned from International competition. Even going to compete in Abu Dhabi is enough for you to be kicked off the national team and lose your Olympic roster spot. A Judo school that even teaches a single MMA class can lose their Federation status which means they can no longer compete, and that none of their students can advance beyond the first rank of Black Belt regardless of skills or knowledge (which is voted on by Federation members).
-Not everyone wants to be hit in the face for a living.
How much does the International Judo Federation hate MMA? They threatened to pull the Judo nationals from Britain (huge event with tens of thousands of expected spectators that had been planned for YEARS) just because the UFC wanted to SPONSOR the event. The heat was so severe that the UFC had to voluntarily pull out as a sponsor.
Rousey pissed off a LOT of people in the Judo Federation as well as old guard Judoka when she switched to MMA. She pissed them off enough that her name is actually modded off of many Judo boards or Judo affiliated web sites. They're not even allowed to mention her name let alone discuss her out of fear that she'll inspire more Judoka to go to MMA.