TMZ Sports has learned over 30 people -- many famous -- penned letters to Judge Shelyna Brown asking her to let Velasquez post bail, which would allow him to mount his defense from home, not a cell.

In addition to the UFC honcho and the guys above, others like Bellator boss Scott Coker, Bellator heavyweight champ Ryan Bader, UFC contender Kelvin Gastelum, UFC lightweight contender Islam Makhachev, UFC featherweight contender Yair Rodriguez, former UFC flyweight champ Brandon Moreno (and many more) sent letters to Judge Brown.

Unfortunately for Velasquez -- who was charged with attempted murder, aggravated assault, and other charges -- it doesn't seem the letters swayed the judge.

Brown denied Cain bail, saying his alleged actions were "reckless," and the "risk is too great" to allow the future UFC Hall of Famer out of county jail.

But, it's the reason behind those so-called "reckless" actions that has motivated so many people -- from stars in the MMA world to ordinary people -- to demand Cain's release.

Velasquez is accused of attempting to kill a man named Harry Goularte ... after the 43-year-old was arrested for allegedly molesting a member of Cain's family -- possibly multiple times.

Now, barring the prosecution changing its position, Velasquez will sit in a jail cell until at least April 12, when he is scheduled to again stand before the judge.