I will speak honestly and say that drawing from my own experience, it is because people feel duped.
When Musk came to public notice he was a fast-talking, confident, seemingly self-made rich tech autodidact who had personally created industries through sheer force of will and a vision.
He talked to a certain type of person who grew up with sci-fi and optimism for the future through technological advancement, only to see a disgusting anti-intellectualism and fatalism take over during the Bush Jr. years. He really spoke to me and I thought he was the real deal.
Then, slowly and over the course of years we started learning that he was a charlatan who's success was the result of luck, connections, or usurpation. He promises and promises and a lot of the big promises turn out to be laughably impractical or unrealistic to the point that no person learned in the applicable fields would take it seriously.
We learn about the subsidies that underpins the success, we learn about the terror that he puts his employees through, we learn about how he treats his family, and how he uses real people's lives and serious events as ways to increase his publicity and image with no concern for any negative effects on others.
The real 'ah-ha' moment for me was when he accused the British diver who risked his life and career to rescue trapped children in an underwater cave of being a 'pedo', because they didn't want to use his useless submarine idea.
And all the while, whenever he announces something, the fans and public rant and rave about a new revolution -- "vacuum tube transportation", "going to Mars in 5 years", "exactly like trains, but instead, cars!", etc. When we mention the problems with these ideas, and that Musk isn't really the best person to be advocating things that he knows nothing about, the fans retaliate with personal insults. "You are just jealous"... etc...
And then when he starts to lose popularity he turns 'free speech' and starts catering to a very unpleasant type of crowd (in many people's opinion). When you start appealing to the right-wing because your reputation took a nose-dive, then you are, um, "not a good person".
After a while it becomes a binary thing -- Musk == bad. I have to stop myself from thinking that way, but every time there is a raving fan talking about the new thing he is a genius at, it rears its head again.
Hope this helps you out a little.