In the Star Trek: Voyager episode "Resolutions," what was it about the planet that afforded them protection from the virus?

This is never explained in detail. From the Star Trek Memory Alpha Wiki:

It appears that the only way the two officers can avoid developing the
deadly symptoms is to remain on the planet, where an unknown element
is able to protect them for the disease's effects.

Ultimately a cure/serum is provided by the Vidiians.