More glass, more room for light loss. Roof prisms also require phase correction coatings on the roof face of the prisms to correct an inherent aberration. The best roof prisms have overcome the drawbacks of the design, but the result is that they are extremely expensive. Quality roof prism binoculars are generally waterproof, an important consideration. They also produce an apparently larger image.
Some people simply prefer the narrower body shape.
But don't get on the roof prism bandwagon until you've compared them for yourself. There is a perfect binocular. The myth is that somewhere out there lurks the perfect binocular. The truth is that there are many perfect binoculars. What is perfect for me, however, may be too heavy for you, not bright enough for another person, and have the focus wheel in the wrong place for the third.