Different type of business... cement typically moves from only a few locations to only a few customers... so the fleet of cars becomes basically special purpose or somewhat dedicated, almost like a unit coal train.
Covered hoppers are also a lot more specialized than they appear, from everything to pneumatic blower connections to the size and configuration of the roof hatches.
I had a buddy that designed a fly-ash system at a power plant, ash was to be hauled away in covered hoppers. A lot bigger deal than everyone involved first thought, specifying or at least trying to work with the particular railroad to determine what configuration was most common for them, and then, how many cars might be readily available.