1. The geography of the place. If Rohan was to attack Gondor it would have to either. Take Minas Tirith, something I find quite impossible. Or send an army over Isen, Adorn, a long way around the Ered Nimrais and back trough eastern Gondor. A trip of estimately 900 miles through unhabited lands.
Gondor on the other hand could easily launch attacks through Anorien without risking too much. I believe cavalry isn't the best force when you fight in woods. (correct me if I'm wrong)

2. Manpower. Gondor had a far higher population than Rohan. Tolkien estimates Theodens army to a little more than 12.000, If all the men of the Mark were ready for battle it might have been something like 25.000. Gondor would have more than twice that number when you include all the men of the southern fiefs.

3. Defence. Rohan had two strong fortresses, Dunharrow and Helms Deep. Gondor had Minas Tirith, probably several in the vales of Ered Nimrais and of corse Pelagrir and Dol Amroth. Tolkien never mentiones that Rohan ever used any siege weapons and if they did, the art would not be as developed as in Gondor.

Other reasons may come as well,but it's late and I'm tired.

Maglor