Several reasons.
1) They're good cameras. Built like tanks.
2) They're famous student cameras (along with the AE-1 and the FE).
3) High availability of very good, very cheap glass. Canon and Nikon in many instances share mounts with their newer AF bodies, which drives up the price of their lenses. Because their AF lenses are so ungodly expensive, many people clamor for older compatible lenses. While all non-screw-mount Pentax bodies are compatible with any K-mount lens, the market for glass is smaller and therefore cheaper. This adds up to a more affordable multi-lens system for your average broke student.