User blog comment:DeadpoolN7/Why didn't the Alliance try this earlier?/@comment-4961148-20130602212324

Probably for the same reasons that real-life militaries don't tend to equip their soldiers with the best arms and armour on a regular basis (e.g. the US Army's refusal to adopt DragonSkin over the current Interceptor Body Armour system). Cost, current contracts, and simple politics.

Who knows how much a M-920 Cain costs? And what benefits it has in conventional combat are easily achieved with more common and selective weapons like rocket launchers.

Also, why not capture as many Reaper Blackstar launchers as possible, and use those? Probably because equipping grunts with unfamiliar, incomprehensible alien weaponry is a recipe for disaster. It's rather curious that Shepard can even fire that thing.

In short, the Alliance, like almost all the galaxy's military forces, made the common and easily understandable mistake of preparing weapons and defences against an enemy that would conveniently fight according to scenarios in which those munitions would be effective.