Use the (mass * acceleration), Luke

I had caught the end of the first Star Wars movie (Episode IV, the first good one) today on Spike. You know what really bothered me, after having seen it again? In the trench run, when the rebels are trying to blow up the Death Star... how come no one has lasers on the back of their fighter ships?

You'd only need one to give yourself cover from the people tailing you:

BAM! No more Vader on your ass.
I'm not a physics major, and I'm just an average Star Wars fan with little to no knowledge of the Star Wars universe outside the movies. But I think I know enough of the two to wonder why the heck they've never thought of it before.

With fighter jets on a planet with atmosphere, like (off the top of my head) Earth, it doesn't make sense to have weaponry in the rear. A fighter jet has a limited amount of ammo they can take up in the air (weight restriction). Also, it's entirely plausible that firing weapons backwards will screw up with a plane's aerodynamics and send it out of control and crashing (a bad thing).

But this is a battle in space. Weight is a non-issue, and there are no aerodynamics to speak of. And since they're using lasers, presumably ammo isn't even an issue.

So with weight and aerodynamics ruled out, all that's left is for the pilot to correctly aim behind him while flying forward. That would be pretty difficult, if not impossible, for a person to do. But the rebels were using droid co-pilots. How hard would it have been for Luke to just say "Hey R2, I'm gonna concentrate on blowing up the Death Star... you go 'pew! pew! pew!' behind me. And take care of Vader for me while you're at it."?

They can create planet destroyers and hyperdrives, but the concept of firing behind them in a space dog fight still eludes them.