Just look at the case/death counts, they are RIDICULOUSLY low in Southeast Asia. This is despite SEAsia being poorer and less stable than any of the NEAsian countries

Bat populations are higher, and bat diversity is higher in SEAsia. Also the custom of eating bats, while rare anywhere, is more common in SEAsia than in China.

IMO Sars-cov-2 started out as a mostly harmless virus, circulating in SEAsia back in 2018 or early 2019. Eventually after reaching China, it mutated (or was modified if you're into conspiracies) into Sars-cov-2. Then right upon reaching Europe, it mutated (or was modified) into the even more deadly D614G mutation