Actually, at this point, the reason might not be the stadium.
Rydon is a ground type, which poison wouldn't have a lot on.
It wouldn't work a lot if the Rydon is already have a status ailment, (such as poisoned, sleep, burned, freeze) you can't put another status problem on it. A Pokemon can't have two status problem at the same time.
In addition, Stadium can actually affect a Pokemon's poison status. Pokémon inflicted with poison is protected from other status conditions and does not wear off over time. Normal poison can be inflicted by attacks, abilities, or by a Pokémon ending their turn in a poison bog.
