IIRC, the issue with plumbing is often sewage volume from cooking + showers and the like. There's a certain amount of load expected per floor of commercial space that is lower than for residential, and often commercial buildings literally don't have a hookup to the main sewer line large enough to accommodate residential use, and adjusting that can be immensely expensive.

It's also a question of what the trunk water/sewer line can handle. There's places in some cities where larger apartment buildings have to hold sewage in a tank and pump it out at night because the city pipes can't handle a normal load during normal hours.