Imagine one company that owns solar city, tesla, spacex and twitter at the same time (and any other if he owns), they are still different companies, just owned by one big larger company, which would really work out better for him and he would have easier time handling it from one headquarters instead of going into each of them.
Elon Musk named sole director of Twitter, dissolves board
Source: https://thehill.com/policy/technology/3712742-elon-musk-named-sole-director-of-twitter-dissolves-board/
So whats up into his mind? Or just simply he doesnt really want for any board and would just tend to run the company alone.
This is indeed an inevitable thing to be done by a company owner.Well, its his decision.