... because not every hotel guest is an adult. I know people that travel with children who ask exactly the opposite question: when in a hotel with a shower only, they view this as a major shortcoming of the room and ask why anyone would NOT want a tub in their bathroom? From their perspective, a bathtub is an essential daily utility to keep their kids clean, and hotels that lack them are doing so to cut corners, rather than cater to business travelers.

Myself, I much prefer a walk-in shower to a tub/shower combo. Many higher-end hotels seem to provide both. Some business-oriented budget hotels are ripping out tubs and putting in walk-in showers (for example, Hyatt House). But for hotels that have lots of families, I think tubs are here to stay.