[Eomer of the Rohirrim]

Maybe there was only so much he could do with two characters. In the other half of the story you had Merry/Pippin, Aragorn/Legolas/Gimli, all the Rohan characters, and later on all the Gondor characters.

There was an awful lot going on in that part of Middle-earth in those few days, whereas all Frodo and Sam were really doing was walking along a road.