It's like censoring your teddies for a man in islam. According to some (pretty standard) interpretations, navel to knee is privates for dudes in islam. Modesty type shit. That's why it's a bit of a curiosity for me to see nominally devout muslim fighters with their whole upper body exposed. They wear the trunks to below their knees like Khabib but it's still not in keeping with the expectations. Believe it or not, it's not just for the women. THAT SAID, I obviously watch MMA and I am not religious but it is what it is.
(mods don't delete just answering question)