Because I heard he was going to have some advisory or executive capacity on government operation. There's a significant conflict of interest there if he's running and owning these companies at the same time.
Its hard to determine a conflict of interest when the role isn't clear though, and the problem there is that everyone can really go off of what they heard through the grape vine.
If the role truly is advisory I wouldn't personally see that as a conflict of interest. Regulators are often asking for advise from those they are meant to regulate without it getting flagged as a conflict of interest (for better or worse).
It used to be that even a WHIFF of conflict of interest was treated as "no smoke without fire, better divest".
Carter placed his peanut farm in a blind trust to avoid precisely that - sadly, we have seen a complete erosion of norms, standards, and morals in public life.
I don't think it's that hard to determine. He has big companies involved in significant regulatory actions and oversight, he would stand to gain a lot by influencing things slightly in his favor. Sure, taken to absurdity everybody in government has a conflict of interest because they are alive on the same planet and have heir own views on things, but for the case of someone like Musk it's pretty clear.
Politicians and bureaucrats can and should consult with the people they govern of course. The "proper" way to do that would be via reasonably open and transparent process that is open to interested parties so competitors, customers, unions, scientists could have their say.
Again I'm fully aware this isn't how things actually work, so I'm not saying Musk is really doing anything outside the norm in American politics by buying a seat at the table. He's just being slightly more open about it than most of them.
I wouldn't be opposed to going after such situations as corruption or conflict of interest issues, but that's going to be a big can of worms.
From the FAA and Boeing to multiple health agencies and pharmaceutical companies, there are a ton of advisory type roles that involve industry leaders "recommending" policy. I'd be surprised if Musk ended up at the top of the list when sorted by impact, counted either by financial impact or number of rules and regulations impacted by industry.