I know many disagreed with the decision, so why did the democrats decide to blame the COVID on Trump instead of Global Warming ?

Conservatives like you misrepresent democrats actions.

Democrats blame Trump for his incompetent handling of the virus from the beginning!

cause people like to blame trump for stuff

Because the Dems listen to scientists and know that the COVID virus does not go away when it gets warm out or that it is no worse than the flu and nothing to worry about.  Also the Dems do not contradict the learned advice of their own advisors and spout pseudoscientific babble about unproven and potentially deadly treatments or praise witch doctors who believe that the virus was caused by alien sperm.

Because Dems are smart enough to know how the world works... (And how bad leadership can impact those things.)

They really should blame China for both COVID and Global Warming.

a)  The overwhelming majority of world's pollution is from China.

b)  COVID is exactly from China.

Trump  did not cause either of those