This is a charged topic, but I'll try to answer objectively without trying to argue whether the reasons are good or bad.

I can think of two reasonable reasons why someone wouldn't want to investigate the matter:

1. It would be embarrassing for China.

2. It may be hard to properly communicate or convince people of the difference between an accidental lab leak vs. an intentional lab leak which could add fuel to a fire of jingoism/nationalism in other countries.

Again, I'm trying hard not to make a judgement here. Hopefully I did a decent job.