Plants don't seem as immediately threatening as fire or water blasts, so that makes it less cool. Also there's less bipeds, which are more popular.
Venusaur are Torterra are bulky-looking quadrupeds, which are just plain not popular.
Meganium and Serperior are both not bipedal, and are comparatively girly looking, which hampers popularly since starters mostly get popular by kids thinking its cool (the exception seems to be if you go all out with the girlyness like with Braixen and Primarina).
Chesnaught is awkward and fat, so it's immediately btfo by Greninja, which is a ninja. Kids and autists love ninjas.
Sceptile and Decidueye are the only traditionally cool Grass starters. And the former has the problem of being the same gen as Blaziken, a traditionally cool Fire starter that GF shilled at the start of Gen III.
