I imagine part of the problem is FFG making do with what they are given by LFL, also. Since LFL has to approve content out of FFG, they may have a say in what is done. It's also a matter of content- what heavy bombers does the Empire have with turrets on them? Also, which faction has the largest concentration of canon fodder fighters that are easily destroyed for the sake of making the heroes look good?
Another part of it is design decisions by LFL for the past few waves when Lore didn't provide something for them to do by priority. The TIE Interdictor/Punisher was FFG's attempt at buffing poorly-performing ordnance at the time, and the TIE Aggressor was FFG's gesture to bring turrets to the Empire. We can debate if that's what FFG should have been focusing on at the time for the Imperial Meta.
