The left wasn't denying the possibility that it leaked from the Wuhan lab. This was a theory very early on and it wasn't "denied" more that it wasn't substantiated.
It became political when it was asserted by people on the right, without any actual research or investigation, that was an intentional release from China.
Majority of that blame falls on Trump's inability to understand or articulate nuanced topics, particularly within geopolitical contexts. Instead of figuring out how to defeat the virus, majority of their energy went into pointing the finger at who to blame--a far easier task than being an actual leader.