A few possible explanations:

- PKD wrote tonnes of short stories. Four big volumes worth, I believe. Short stories are great for movies because they present a central idea but give the director lots of freedom.

- His stories frequently involved several tropes that work well in movies: drugs, massive corporate entities, multiple realities, and the everyman hero.

- Other sci fi authors had a tendency to be preachy (Heinlein), or too progressive for the time (le Guin). PKD wrote in an unassuming and non-judgemental way. His characters might have had regressive opinions, but he never seemed to push what he thought was best for society.