How would a Omni like the Gun fair trying to get those BA into a position close enough to jump off the attack?
If you're going to invest in a 'mech (much less an Omni) to move BA, that means you're expecting them to do it the most opposed situations imaginable. That skews the requirements to armor and speed. Weapons are a distant third.
I think there are different uses for Omnimech BA deployment.
The Owens/Firemoth are designed to do a high speed insertion ahead of other forces using the BA to claim ground.
But the Gun isn't fluffed for that use.
Its an "Infantry Support Mech". IE, the Sentinel, Scorpion, Vulcan, and several other "less than stellar" designs over the years.
As Glitter noted above, Quads, Assault BA, stuff w/o X# of hands/claws can't even use Omni's. Some stuff requires Vees.
Some missions require speedsters.
But sometimes, you need something that carries BA in support of other mech forces & doesn't need speed.
Like, a lance of Guns moving up BEHIND the Heavy Mech Line, keeping pace with 4/6+5/8 mechs but not flying out in front of them at 10MP+ movement.
Something that waits to deploy the BA till after the battle has started & the enemy is focused on the big boys. That could be a Gun moment.
Or if you are doing that "Infantry Support" job and you need to get the BA somewhere that a Vehicle wont go, like on top of a mountain, or, on an airless moon, etc etc.
The Gun is niche to be sure, but its not useless, I think you would see it in some of those Militias/Guard units that started to get issued BA & Mechs instead of just leg infantry & light tanks.