Well, I think your reasoning is pretty spot-on. Most people go to Pokemon for the fantasy aspect of the Pokemon - like the ones based on myths and legends, or the ones that are just made up - or the cute ones, in which a lot of them are based on animals. As the result, the ones who are based on animals, but aren't part of an evolutionary line, will kinda be pushed aside, both by the fans and the developers. I mean, I reckon Tauros was pretty popular back in the days of the first generation, considering Ash caught like 40 of them, and at that point in the anime they seemed rather strong. And Whitney's infamous Miltank kinda speaks for itself, but despite how strong it was during the battle, not many people appeared to actually have any interest in raising one. Same king can be said about Norman's Slaking.
They all kinda shone through in the battles, but weren't really anything people were interested in having, and with all the added Pokemon, they're just kinda left in the shadow.