I mean it wouldn’t be that hard to mount a pair of bushmaster 20mm Cannons on the back of a flat bed HUMMVE for fire support roles

Why would we need to, though? Sure, a technical is pretty bitching, but the platforms we produce already do the job better. The only thing bolting some more guns to a HMMWV would do is make a more ineffective platform than the shit we already have. A Bushmaster is 25mm, BTW, and it's gigantic. You want to bolt up two of those to a flatbed HMMWV? They can barely hold one, much less two.

Technicals exist to cheaply and economically fill a role that we have better examples of in our military. We make better things than old Toyotas with guns hot-glued to the bed. Why would we want to move over to less effective weaponry?

Is it simply that a Hilux with a bigass cannon is sexy?