It's hard to prove why something didn't happen, however there are a few reasons the Columbian Exchange wasn't entirely equal. The biggest reason would have been a lack of widely used domesticated animals in the Americas. On the other hand, you could find livestock in essentially every community of the Old World. Viruses love to jump from species to species, and living in proximity to a few different mammals means that it only takes a few tweaks of the genome for things to become a pandemic. I would argue that the best proof of this is the fact that many of the infectious diseases that decimated the New World continued to thrive in the Old World as well. The Old World had many diseases that were great at infecting humans, and many were able to hang out in reservoir species like domesticated mammals.