system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are UFC and boxing matches the only mainstream sporting events where fans need to purchase PPV to watch it live?
gpt
These sports are organized in a kind of ad-hoc way. There's no structured "season" guaranteeing a certain number of fights at certain times. Instead, promoters organize events based on who they can get and when. 

It's difficult to sell that to a traditional TV network. They have schedules to keep and prefer to broadcast roughly the same thing every week. Instead, you sell each event directly to the public via PPV.