Rank 2: Restricted Breathing

Pescryodon completes the quartet of Pokémon from Gen 8 that can be revived from two fossils. The Water/Ice Pokémon looks like it would be a skilled hunter in the water. However, somehow, it drew a big losing card in the evolution bingo.
According to the Pokédex, the pocket monster supposedly went extinct because its breathing was restricted. It does not help that no attack could injure its facial skin if it could not get enough oxygen as a result. Many Pescryodons may have suffocated, making revival essentially pointless.
Rank 1: Stupidity

The saddest reason why a prehistoric Pokémon had to die is its own stupidity. Rameidon’s strength is its hard skull. It is said to be able to tear down entire jungle trees, to be as hard as steel, and to prevent a knockout when two Rameidon butt heads.
However, the gigantic skull is also its biggest weakness. Although there was a lot of space, Rameidon only had a small brain. This led to the Pokémon being stupid and thus going extinct. This is stated in the Pokédex entry from Pokémon Ultramoon.
None of the prehistoric Pokémon looks particularly tasty. In contrast, there are other pocket monsters with which we would gladly fill our bellies. And yes: some can actually be eaten! The tastiest specimens from the Pokémon world can be found in the ranking on MeinMMO: The 5 tastiest Pokémon in the ranking