I don't think it is because of the business aspect; Boeing in any investigation won't have leverage as the investigations are run by the local equivalents on the NTSB. To the extent there is any leverage, it's the NTSB that might have it, but it would be informal. The Chinese government is stonewalling this because they deem it politically inconvenient/embarrassing that they had a pilot suicide/murder take place. For instance see also https://admiralcloudberg.medium.com/the-crash-of-egyptair-fl... for the story of EgyptAir 990.