Actually, Luigi talks quite a bit... in fact, you can barely get him to shut up in PMTTYD. The thing is, your main character doesn't talk. In the M&L games he is as quiet as Mario, because he's a main character. I believe the reason is Miyamoto's desire to have you, the player, put yourself in Mario's shoes (or Luigi's when he's part of your party). As a result, Mario doesn't do anything without your say-so. Note that in Luigi's Mansion, Mario said "Get me outta heeeeeeeere!!!" from inside the painting, while Luigi responded with "......!!" But Luigi made quite a few comments when you'd check things with the GBH. If Luigi got his own RPG he'd be paired up with an original character that talked (if anyone). It's been that way in every Mario RPG:
SMRPG: Mallow Geno, Bowser, Toadstool (no, the two latter aren't new, but they've never been your allies before)
PM: Partners, whichever was out would do the talking
M&L:SS: The only exception...
PMTTYD: Your partners again
M&L:PiT: Stuffwell
Edit: You never said it was an RPG, but I assumed that since Luigi needed a partner it would be. You don't need to talk in a side-scroller, really...