The main reason are the flexibility it gives, and the faster learning curve.
Digital photography has a fast workflow compared to film. Point, shoot and print in a few minutes time is possible. In a real world, it is still fast: take pictures, upload on the internet, order pictures, the printouts are usually delivered within 24 hours, 48 hours at the most (where I live).
Digital photography is cheaper (again where I live: 10 cents for a normal 10x15cm printout if digital, film: the film itself, processing + printing average overall cost is 50 to 70 cents per picture)
Digital photography gives you immediate results: you can judge the result on a screen and re-shoot if necessary (this depends, however, on the quality of the screen on your camera)
Trial and error doesn't cost anything. This is a fantastic learning tool. Try different settings, look at the results, compare and learn by doing. On the other hand, that flexibility can be a disadvantage because some people are not thinking anymore and just rely on the technology.
You can shoot a lot more in general (thanks to the zero cost), increasing your experience level much faster.