Why were many native Americans vulnerable to European diseases

The correct answer is that they have built up immunities!

The illnesses which lead to the death of many native Americans have been present in the old world for a long time already that many people have already died from them. Those that didn't die were immune and passed on this immunity to their children.

I'm just, a king a good guess but the natives did not know that Christopher Columbus had been carried the small pox with him so many of the natives died of the disease