Manufacturers (e.g., Qualcomm, Samsung) won’t return your call unless you buy in huge quantities, hundreds of thousands or millions of units.
Lack of documentation.
Information restricted by NDA.
Non-free binaries required for lots of hardware.
Generally lording over the market and exploiting their position, to the degree of anti-competitiveness, and as a consequence artificially extending the rein of non-free software in the mobile domain.