The standard argument here is that the maintainers of the core technology are likely to do a better job of hosting it because they have deeper understanding of how it all works.
Hosting is a commodity. Runtimes are too. In this case, the strategy is to make a better runtime, attract developers, and eventually give them a super easy way to run their project in the cloud. Eg: bun deploy, which is a reserved no op command. I really like Buns DX.
Why invest into a company that has the additional burden of developing bun, why not in a company that does only the hosting?