We've also found humans, bears, wolves etc. frozen in glaciers. But it was only a very small subset of these creatures: the vast majority carried on breeding - and so too with these viruses. This is a "snapshot" of a virus doing the rounds in a host 15,000 years ago. We will likely already have somewhere out in the world descendant strains of that virus, plus the descendant host creatures it infected. So the hosts have already had adaptation time to that virus.
The other factor is that viruses target certain proteins in bodies to bind to, usually with high precision, like having a key to a lock. Outside the sheer, dumb bad luck that a virus might be able to latch onto a novel target, in most cases a virus with no familiarity to a host (or a close enough related species) is effectively going to a be a key in a house full of the wrong locks, and unable to access anything.
