The price of non-silicon cells I think is largely dictated by the extra processing. I think the hardest thing you need to do for silicon is grow it as a single crystal, but even that isn't 100% required (but it will boost efficiency). And you've got abundant, cheap raw material.

Other types of cells have more complex growth and fabrication processes, often requiring a vacuum, tight stoichiometry/growth control, and expensive/finnicky/dangerous precursors.