Normally because they were playing behind AFL level rucks. Grundy can't beat out Witts. Says it all at the moment.I would say that many 200 game ruckmen were playing VFL in their second year.
Grundy also has less development potential than other young rucks as he's already advanced physically, normally the tall rucks are beanpoles that need filling out, hence they play reserves till the get the muscle. Grundy isn't going to suddenly improve by putting muscle on, and i don't think he's finding an extra 5 inches of vertical leap he needs to really make a step up.
Reckon Grundy will have a long successful career without ever cracking the top 5 rucks in the league.