Musk’s repeated outbursts against advertisers have dried up the main source of revenue for the loss-making company formerly known as Twitter. A recent decision to sue them for heeding his own advice to not buy ads on the platform hasn’t helped. At some point, he will have to provide a fresh infusion of cash to salvage his $44 billion takeover.
Because it will affect Tesla's share price (negatively), just like it did last time, and Fortune's job is to report on things that affect share prices.
There is also a public interest element, because Musk is currently the richest man on earth - which affords him massively undue power and influence - but he won't be if he manages to crash the Tesla stock price with a massive sell-off.
Tesla is exceedingly overvalued right now which makes it a very volatile stock. We already saw the price crater the last time he sold stock to keep Twitter afloat, and it only really recovered when he pinkie swore not to do it again.