They have exactly one V-150 Ion cannon on Hoth.
The ion cannon will fire several shots to make sure that any enemy ships will be out of your flight path. -- Leia in Ep. V.
Note that Leia uses the singular form. This means the Ion cannon is busy, and re-targeting between orbital shots and ground-level shots seems to be too time-consuming. X-Wings were not available for ground defense either since they were needed to escort the transports. The movie only shows the ground battle and mentions the battle that appears to happen in orbit only orally, but does not depict it.
I see no reason to think that the Rebels didn't have ion guns if their arsenal had them before.
The base on Hoth wasn't ready (the Empire attacked soon after it had been established), and the Rebels were in hiding. They were not prepared for an attack by the Empire and had to make due with what they had on hand and needed to improvise. They could probably have brought in more ion cannons if they wanted to and if they had more time, but neither was true. The whole point of the base was that it could be abandoned easily, you don't put in too much resources nto an expendable base.
That being said, the walkers in Rogue One and Ep. V are not the same. The former are AT-ACTs, while the latter are much more heavily armoered AT-ATs.