Because lines tend to trend up over time. You're betting on lines going down, and paying rent while doing so as shorting requires you to rent/borrow shares from somebody else. It's an extremely high risk activity that can easily result in an investor losing a very large amount of money.

Elon Musk is politicized so you're going to have people wanting to short against him, for reasons other than it being seen as a rational and sound investment strategy. This is one reason brokers tend to restrict this activity to certain types of investors who are more able to appreciate the risks, to say nothing of baseline necessities like needing a margin account to cover potential losses. Shorting is just very different than buying a stock.