Complex sentence: Despite the benefits, BSkyB does not promote "Freesat from Sky" ("FsfS") mainly because of BSkyB's pay TV service, Sky, the UK's most popular pay TV service, which BSkyB focuses on.
Split into simpler sentences: Despite the benefits, BSkyB does not promote "Freesat from Sky" ("FsfS"). BSkyB also has a pay TV service which is heavily promoted, Sky, the UK's most popular pay TV service, which BSkyB focuses on.