The reason why (all) fossil Pokemon are rock-type may not be because only rock-types lived back then. Perhaps it is because rock-types lend themselves to the fossilization process better than other types; thus providing scientists a good enough sample to revive?
This happens in the real world. Scientists estimate that only ~1% of species that have ever lived are discoverable through fossils.

We may never know all the species of ancient Pokemon if only ~1% of extinct Pokemon can leave fossils. What do you think kind of non-rock-types that lived long ago that never survived?