It's more that the category of "equivalent vehicles" is somewhat narrow. Mack dominates for garbage trucks because platforms of that size (18'-ish WB) rated for the 32-ton-ish GVW class aren't that common. Garbage trucks and dump trucks are two major applications, because both handle material that is much denser than typical truck cargo (once compacted, in the case of garbage trucks). Dump trucks of course are typically conventional cab, with the step-up cabover being a lesser-used option. There are some interesting design constraints that result in garbage trucks, particularly with pressure to minimize engine volume since putting a step-up cab over a large diesel engine is always a difficult space use exercise (and one that is almost unique to garbage trucks). This ends up meaning that the cabover version needs a pretty distinct frame from the conventional cab version because of the engine placement, and there ends up being implications on the suspension design. Historically some garbage trucks have been step-through, which is convenient for the operator, but extremely difficult to implement on modern truck platforms because the engine ends up occupying most of the middle of the cab. You pretty much have to go to rear-engine, which is rare and not really done to my knowledge except by Oshkosh for their generally very interesting front-end cement mixers.

That was all sort of a tangent, the point is that most garbage trucks are built on a truck platform, the Mack LR, that is specifically a garbage truck platform. I believe I have seen cement mixers built on the LR platform too but Mack doesn't seem to advertise it for anything other than garbage trucks.