I'm not certain whether it's just certain piano manufacturers, who had decided to have some upright pianos to have two pedals (without sostenuto). Some Zender acoustic pianos, for instance have two pedals. Then other brands like Kawai, had a few pianos with 2 pedals while most of their models have 3.
As you may be aware, most, if not all studio/acoustic pianos have minimum two pedals - the soft, or una corda pedal, and the sustain, or damper pedal.
The third pedal (in the middle) found on some pianos, is a practice pedal, which locks in position. Usually this involves bringing a curtain in place between the hammers and the strings, to make it very quiet, even when played hard, in order to be able to practise without disturbing too many others.
On other pianos, the middle pedal is dedicated to sostenuto, whereby it holds the dampers off only the strings that are vibrating, and their keys pressed down, at the time the pedal is depressed - a sort of sustain for individual notes only.
Obviously, a sostenuto pedal will add to the cost of an acoustic piano, and will usually be used by more advanced players. They can be found on uprights, but are more common on grands. So the answer to your question is that most players won't use the sostenuto - it isn't marked in a lot of pieces - so far more 'ordinary' uprights will be marketable.