Mountain biking is well known for being an unwanted presence. It makes hiking trails into miserable obstacle courses. Made worse by the fact that if it's allowed at all, people will go as fast as they want, it's not like there's radar traps set up.

I'm in favor of majorly reducing all hobbies that have danger or inconvenience to others. If you can't play 110DB music in the streets at night, there's clearly a general precedent and consensus that your hobby is not anything special and has no right to be a public nuisance or environmental disaster.