Cause they're Primary Function is to stand in the background fire at some fast moving enemy mecha in vain and then get destroyed like the cannon fodder.
Look at the Defender and Phalanx both just have one weapon. No back up guns or swinging arms. The Monster, Tomahawk & Spartan have a few more weapon option but they are pretty much all Destroids stuck to fighting on the ground. Destroid do look cool but You can take a VF anywhere and they have several weapons to pick from.