I wonder what would prevent them from doing it? Is it a problem with the artists, or some issue with CDPR? I doubt that it's the later, and if it's the former, may be we should reach out to artists directly and ask what prevents from from releasing it on GOG or Bandcamp. I hope it's not some kind of exclusivity contracts. Those can be nasty attempts of distributors to ban competing services.
By the way, what exactly is part of the extended soundtrack? Just singles or more?
It may be a business decision, motivated by profit; however, since I don't know precisely, I'll withhold judgement. As to the expanded content: the standard CD soundtrack, from both the Collector's and Standard Retail Editions, contains 31 tracks. The ones, mp3 and FLAC, on GOG contain 35 tracks (4 bonus pieces). The extended version announced by Marcin is supposed to have 56 tracks (25 previously unavailable), including the two songs.