Because that's what it is! Once Upon a Mattress debuted off-Broadway in 1959, and soon moved to Broadway. It's been a longtime favorite of mine, and many other people.
Several different productions have aired on television since then, the most recent of which was this Disney version in 2005. You know how the major US networks often have productions of musicals during the winter holiday seasons? That was the case with this version of the musical.
So, YES, it plays like a stage musical. That's kind of the point!
