Frequently do small loads. Run just one drawer at a time to use less water than a full-size machine.
Have a galley or narrow kitchen. There's no tip-down door to contend with.
Never want to put away another dish. You can pull clean dishes from one drawer when you set the table, load dirties into the other after the meal.
Want to hide a second dishwasher in a butler's pantry or wet bar. Use just one drawer, and hide it behind a wood panel so that it blends with the cabinetry.