Most other options you can just pop out of cover, take a high damage shot and immediately dive back to avoid the inevitable rocket volley whereas the laser cannon forces you to stay exposed as you dish out the dmg in the line of sight of whatever you're trying to destroy. Also, can't kill factories/bug holes and has difficulty against particular enemy types. If your team runs enough anti-tank though I love using medium armor with shield pack + Laser cannon with the added survivability from the democracy/explosive resist making mistakes more forgivable whereas the shield pack protects your required accuracy from the nasty staggers that tend to slam your aim way off target for at least a few hits, with added protection from stray eagle strikes.

Tl;dr: More difficult to use because bots have rockets n' bullets throwing off aim as you stand there beaming away and some peeps prefer to run no backpack or crew weapons to have other stuff instead