Slides have a much higher contrast range when you project them (that's why slides look so spectacular and real). When you print them on paper, the blackest black still reflects 5% of the light and the whitest white only reflect 95% of the light. But black on a slide blocks the light almost totally and the whites are almost 100% transparent for light, so the contrast range is much larger.
Showing pictures on a computer screen is better, but the sensor of your scanner will also be a bit limited in dynamic range and you lose contrast in the optical system too I guess (not sure how exactly these scanners work).
With photoshop or another editing programme you should be able to enhance the pictures a bit, but you can never regain what was really lost.