Epidemics like smallpox often spread rapidly and wiped out entire native tribes. why weren’t these diseases as fatal for the spanish?

Because the Spaniards were already immune to the disease.

Think of it like this - when you catch the flu, you go through the stages of the flu (Lets not get into that info) but you get what I mean, well anyway, your body gets stronger after it has been exposed and contaminated by the virus/disease. Meaning that it shouldn't come and infect you whenever you are around it with other people infected with it. Yes the Spanish has died from the disease, but after so long it barely even affects them anymore. While with the Mesoamerican tribes they weren't immune to it yet, therefore taking a huge toll on their population count. Now if the native tribes had advance medicines to help cure/weaken the disease then that would be a huge game changer for them and allowing the disease to not get them infected as bad. But unfortunately, they didn't have that perk. So therefore they died.

Hope this answer helps! :)