What yultemsee said. Either the firepower of the rocket launchers should be increased, or regular fire should be less effective. I use the rocket launcher myself, but I hardly see anyone else do.
Maybe if you gave players the ability to fire anti-infantry rockets (larger blast radius but useless against vehicles) in addition to anti-vehicle rockets, it'd be used more. But that would work against how the rocket launcher infantryman is supposed to be strictly anti-vehicular.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.