A lion's mane grows because of hormones, specifically testosterone. This is the male hormone. Its in people too.
The mane is for show, to show all of the ladies that he is a man and able to reproduce. It also helps protect him when he is fighting with other guys for breeding rights.
Once that hormone is gone, which is what happens when he is neutered, he is not able to grow his mane anymore.
A female lion can grow a mane too. It happened recently. She had a hormone imbalance, she had too much testosterone, so she started to grow a mane and act like a male. They have her on meds now to fix the problem.