Native Americans (and indigenous islanders) did not have many diseases on their side to use against the Europeans. Plagues spread quickly between people. Secondly, if you get a plague, either you die or you become immune. If you survive, you will never get it again, because now your body knows how to kill it. However, you can still carry the plague, and you can spread it to other people, but it can't hurt you.
The reason is that the New World didn't have plagues! But why didn't the New World get plagues?
Plagues prosper when you have a civilization/city which does a bad job of separating sewage from drinking water. This can help spread the plague, which helps evolve it fast. This makes deadly plagues. One such city which had bad water management was London. This made Cholera, one of the deadly diseases which haunted the Native Americans.
Cholera, and many other plagues, cannot survive in an isolated area. It will simply infect everyone, and then die because it does not have any more hosts available! The remaining living people will be immune, so the disease will die off. So, plagues need cities and highly connected trade routes to survive. China and India were great for plagues to live, because they had extremely high populations for most of history. In cities throughout Eurasia, people migrated, people were born, it was crowded, and that meant that the plague could spread indefinitely and prosper.
So, since the New World had less cities (and less population), it had less plagues. The New World could not ever be as advanced as the Old World (before Europeans) because they did not have the animals that we depend on. Pigs, horses, cows, elephants, and dogs were vital to the rise of humans, and they were not there in the New World. That meant that the New World had less cities, less population, less achievements, and less diseases.
However, the New World still had connections, and cities such as Tenochtitlan. The reason that there was no diseases coming from Native American cities is because the germs that cause the plagues don't want to kill you. Just like how the common cold lives in you and keep you alive, plagues want to keep you alive so it can live in you. The reason it kills you is because it thinks you are another animal, like, a cow. A germ in a cow, that does not kill it, accidentally traveled to a human. It does the stuff it does to a cow, and you die. Now it spreads to other humans. Now, lots of humans are dying and you have a plague.
This means that to have a plague, you must have as much contact with animals as possible. In the Old World, where farmers tended for cows and pigs, while warriors rode horses and elephants, there was a much higher chance of accidentally getting a disease from the animal that in the New World, where there was not as much contact between animals and humans.
This is what happened with the coronavirus, when a human came in contact with a bat disease. For a bat, COVID-19 will be easy to beat, just like the human cold. However, for humans, it can kill you.
So, the short answer? There were more animals to domesticate, so there were diseases hopping from animal to human. These diseases prospered in crowded, well traveled cities, and high human populations made more diseases. Also, bad water management and sanitation helped diseases spread. America did not have domesticatable animals (other than the llama) and less cities and less population, so there were naturally less diseases.
That said, there were some diseases that spread from America to Afro-Eurasia. Syphilis is a sexually transmitted disease that is believed to have originated from the Americas. However, most of the diseases did come from Eurasia.
There is one missing piece to this answer. The Vikings did come to the Americas far before Columbus, and it is likely that the [Mali Empire (Abubakari) also made it.]  There is a small chance that China (Zheng He) also made it to the Americas before Columbus.(2) So why didn't the natives get the plague from these early travelers and be immune to the Spanish? The reason is that, when the explorer landed on the coastal tribe, he would have spread the disease to them. However, the plague could not have spread to any other tribe due to lack of contact, and due to isolation. That one tribe would have died out, and the 5% that survived would be immune. This is kind of like social distancing. However, the disease would die out because it would have no one else to infect. That is why most of the Americas were vulnerable.
Check out this source: https://www.youtube.com/watch?v=JEYh5WACqEk
Hope this answers your question.
(1) http://moorishharem.com/the-battle-for-the-americas-mansa-abubakari-ii-181-years-before-columbus/, https://www.csu.edu/dosa/AAMRC/news1.htm