This would not be a problem if, say, the walkers were extremely fast and maneuverable. For instance, in the real world, jet fighters have all of their weapons pointing forward, too. But they also go faster than the speed of sound. These bastards, on the other hand, have the top-speed of a Geo Metro driving across a river. It takes them what looks like an entire day to execute a full U-turn. That's good news for you if you're Luke Skywalker, because if you get within 20 yards of one, it can't hit you to save its life.
Years of additional weapons development actually lead to shittier war machines.
Also, the Empire seems to have constructed the Walkers without considering that most militia battles are not fought thirty stories above ground, so the tank commanders spend the entire day on Hoth with their chins to their chests trying to see what the hell they're supposed to be shooting at.
The only reason the rebels had such trouble with them is because the speeders insisted on using what Luke referred to as "Attack Pattern Delta", which appears to mean, "fly directly at the enemy in the one single spot where they are able to shoot us."