Why did this custom of watching gladiators fight become so popular in Rome?

The games were so popular that successful gladiators could become extremely rich and very famous. As a result, while most gladiators were condemned criminals, slaves or prisoners of war, some were freedmen who chose to fight, either as a way to achieve fame and fortune, or simply because they enjoyed it.

the answer is solomon northrup. he was a free-born african american, abolitionist and professional violinist that was drugged, kidnapped and sold as slave in 1841 and remained in captivity until 1853 where he regained his freedom with the of the then-governor of new york, washington hunt. he wrote and published his memoirs "twelve years a slave" in 1853, and in this text, he tells the story of his escape thru one of louisiana's swamps (the great pacoudrie swamp) in which he ended up covered in green slime from crossing the surface of the dead water.