Where was Gondor?
So was King Theoden not aware that Gondor had been involved in increasingly hostile Mordor skirmishes for many decades? Gondor's armies were dwindling, and they kept losing ground to the orcs. They could not have spared an army, even if they wanted to.
Not to mention a good chunk of Gondor's citizens lived on the south side of the White Mountain range, and it would have been quite an effort to rally them all to help Rohan in its time of need, besides the distance going around the mountain range to get to Edoras, the Westfold, or Helm's Deep.
Gondor wasn't just Minas Tirith like some may think. However, those living south of the White Mountains (at least in the books, in the movies it's a ridiculous soapy green undead army) would later be used by Aragorn to assist in the Battle of Pelennor Fields. So I always found it a little odd, at least in the films, Theoden seems unaware of Gondor's commitment to keeping Mordor at bay.