In the space battle above Scarif, three rebel attack fighters launch ion torpedoes at a Star Destroyer.
The torpedoes completely disabled the Star Destroyer. They knocked out the engines and the disrupted the energy distribution systems within the ship so badly that it could fight or defend itself.
Those same fighters could have fired ion torpedoes into the ring shaped space station that made the planetary force field.
Why didn't the rebels use ion torpedoes to destroy the shield gate?
I prefer answers with quotes from canon sources (screenplay, novelization, or comments by producers).