Because it's cost prohibitive to find every animal on the earth that has the virus and medicate or eliminate them.

Humans already eradicated a disease (I think it was a virus). It was done with a vacine of an altered version of the virus. The vacine itself was a virus which could infect others and spread among the population.

So, you don't really have to find every animal. Most should be enough.

There have been efforts to put vaccines into treats that various animals would eat. However not all vaccines can be delivered in this way, and even of those that can, getting the right animals to eat them is tricky.

It is fairly efficient with rabies. Aside from bats, most European wildlife is now rabies-free. But to pull this off worldwide is not realistic.