I think Left 4 Dead isn't brought up as much simply because people might think:
- Heavy and TF2 in general is cartoony and more funny, thus it's better suited for Smash.
- And Half Life is such a revolutionary title for the FPS genre and Sakurai himself has praised HL before.
- And in general, L4D's protagonists might be seen as generic?
Those are the only plausible explanations I can think of. And I don't think it's because it's a gritty zombie game because Resident Evil is seen as a very likely candidate (Putting aside the spirit event).
But yeah I would dig L4D, I love its atmosphere. Imagine the stage. It could take place somewhere like in a bunker or a train station and you could have the different types of zombies be hazards. It would be really cool.