There are mirrors available for most SLR cameras, but their limitations make them fairly special purpose instruments.

1. Most use catadioptric mirrors, which have a central obstruction that limits the minimum focal length that can be used -- it would be very difficult to keep the central obstruction small enough for the focal lengths most photographers use most of the time.
2. The central obstruction leads to out of focus highlights being "donut" shaped, which is often deemed unattractive.
3. A camera lens normally needs an adjustable aperture, which is relatively difficult with a mirror.
4. mirrors typically give relatively low contrast compared to lenses.
5. The primary reason to use a mirror in the first place is for really large apertures; you can support the back of a large mirror, where a lens can only be supported at the edges. Almost no camera lens is large enough for this to really become an issue -- for example, a 600mm f/4 still only has a ~150 mm (6 inch) aperture.

Edit: @Marc raises a good point: to be at all fair, I should probably point out some of the strengths of mirrors:

1. Catadioptrics are usually quite short for focal length (thanks to folded light path).
2. Usually quite light
3. Often Inexpensive (especially used -- and they're often barely used).
4. A pure mirror (with no transmission through glass) eliminates chromatic aberration.