The thing is that the Rifle (Cannons)s are so archaic that they’re not designed to deal with later, tougher forms of armour: when dealing with units (not counting BA) whose armour has BAR7 or higher (which includes Primitive ’Mech armour and all more advanced forms), each gun deals 3 points less of damage. The much-maligned AC/5 masses the same as the Heavy Rifle, hits almost as hard, doesn’t suffer the same damage-penalty, and has far better ammunition endurance for every ton of ammo it carries.
That’s not to say that Heavy Rifles are completely useless: like all Rifle (Cannon)s, they’re cheap, easy to make and supply, they do deal damage the enemy, and a solid hit from one will get the attention of most BA suits (which don’t enjoy the -3 damage-modifier that heavier units get). It’s just a niche weapon that typically speaks to the user lacking a tech-base advanced enough to deploy anything more potent.
Of course, if you want to play a campaign in a Tech Level C (or even TL-B) environment where Primitive ’Mechs are fighting it out with, say, units from TRO:1945, the Heavy Rifle starts to look awfully powerful as a main-armament.... }:) ;)