One-tape adult diapers simply don't work because the diaper is too big and undergoes too much stress in day-to-day adult life.
Essentially, the multiple-tapes system allows the wearer more comfort, as they can adjust the tightness of the diaper at varying points. With the single tape, it's only secured at one point and doesn't quite have the adjustability as a multiple-tape ones - this also leads to more stress on each tape. There are single-tape adult diapers out there; they work because it's more elongated and wider than tapes on other diapers, however, the reports on them have usually been negative. Mostly that they don't hold up and the overall quality is rather lacking.