If you want an actual answer, it's a combination of genetics and culture.
Native American populations had severely limited genetic diversity. Far lower than Africa, Europe, and Asia.
The native diseases in the New World were ones that European immune systems were able to adapt to pretty easily. Keep in mind, all living Europeans were the descendants of survivors of every plague you can think of. Hundreds of diseases out of Asia, Africa, the Middle East, and dozens of populations in Europe itself.
That lead to a highly adaptive immune heritage.
Native Americans had no such diversity of immunity, and all had very similar immune heritage. What one was vulnerable to, most were. Where Europeans had a much more diverse ancestry.
On top of that, European culture had a tradition of isolating the ill, going back to at least the Black Plague, while most Native American cultures were more likely to have friends and family spend time around sick people to comfort them.
Which sounds more friendly (because it is) until you realize that being in the same room with someone infected with a deadly disease is a good way to catch that same illness yourself.
So, a combination of genetic and cultural and historic factors.
Europeans were walking disease clouds of all the stuff their ancestors had survived. Chicken pox is annoying to most Europeans, but ate the skin off of Native Americans.
Native Americans had diseases of their own, but European immune systems, that had survived plague after plague after plague for thousands of years, tended to laugh at those germs and beat them up for their lunch money.