It's labeled as racist because Trump said it first.
Now that trump is gone, we can go back to a normal situation where we discuss any theory without being called racist.
Because Trump said it. And trump doesn't exactly have great credit when it comes to not being racist against Chinese people.
So, we label this theory as conspiracist and racist.
I don't think i have further explanation.
I believe (with no way to prove it) if Trump had, with his trademark tone and delivery, blamed China's Wuhan Seafood Market and thrown in a few choice insults about China, that theory would have been the one Big Media shut down and filtered out. Instead it was the lab leak theory.