I guess it's because the Bulbasaur line (particularly Venusaur) doesn't have that "cool" feel to it, like the Charmander or Squirtle lines. That said, Bulbasaur is by far my favorite of the Kanto Starters, if only because it's the only one with a legitimately creative design. In terms of appearance, Squirtle's just a blue turtle and Charmander's just an orange lizard. I can see why Blastoise has a lot of fans, though, but nothing about Charizard strikes me as particularly interesting... But, this is a thread about Bulbasaur, so enough of that.
Also, Bulbasaur has way more fans than Turtwig does. Coincidentally, Turtwig is my favorite Sinnoh Starter, but that statement seems false...