Rain as a whole is a very...unstable....archtype. For example, you will see baraskewda dropping and raising, because it is really only used on rain teams. Because of this, seismatoad dropped. On top of that, it has never been a "premier" rain abuser, as there are better, like ferrothorn to easily take fire attacks.
I am right about it potentially being a rain abuser, but it is a defensive Pokemon. The reason it dropped is because of the popularity of rillaboom. Rillaboom absolutely destroys this mon, along with other common Pokemon like kartana.