Complex sentence: The film, understandably, has developed a cult following, also because of its artistry and length, but mainly because it isn't available on DVD (only bootlegs transferred from old VHS sources exist).
Split into simpler sentences: The film, understandably, has developed a cult following. In part due to its artistry and length, but mainly because it isn't available on DVD (only bootlegs transferred from old VHS sources exist).