Fair question -- always question assumptions -- but no, that doesn't work as an explanation, I don't think, because Romans in the north could have made bread in the same way that it was made all over the Roman world. So why go to the fag of making beer, as part of that process, in the outlying provinces only?
Maybe in the outlying provinces, where soldiers were on garrison duty, individual soldiers had to make their own bread, but to get the bread to rise, they had to use yeast, which was too difficult for individual soldiers to keep active. Therefore, soldiers lit upon the solution of using the local yokels' smelly alcoholic beverage (glorious beer to us barbarians) for baking bread, and thus great quantities of the stuff were kept on hand for soldiers on the frontier. Also, maybe (in a pinch) they drank some too.