It also shows a problem CD Projekt and GOG have. The lack of advertisement. I've just read a case where someone bought The Witcher 3 on Steam because he didn't know that GOG is part of CD Projekt (YES!). He would have preferred to buy it here on GOG because he could have supported CD Projekt. But it was too late.
GOG's main problem is that people associate PC gaming with Steam. There are many trailers, news, etc. on different gaming sites that only mention Steam although a GOG version of a game exists. They usually post links to the Steam store. I know some people who prefer buying DRM-free games, but they end up buying the Steam versions because they don't know that a GOG version exists. And this is most likely also the problem with the GOG version of Supraland.
Personally I think all that you wrote is 100% correct.
As for the issue of GOG having less publicity and coverage than steam, without trying to enter into any "conspiracy theories", I guess it's safe to say that a heavy part of the videogame industry (mostly the AAA type) has no interest whatsoever in people knowing that there's a DRM-free market for videogames. It's more and more evident, as AAA publishers seem keen to enforce stricter DRM on its products.
The gaming press is also (on my opinion) one of the greatest culprits on this matter, since for many years they always helped to enforce the definition that "steam = pc gaming".
I still remember well how it wasn't until 2013 or 2014 that Rock, Paper, Shotgun
began to acknowledge the existence of GOG. Because, for all the previous years, whenever a game was launched (on both stores) they'd only mention the steam release as if no other stores existed. Until increasing pressure from the readers forced them to finally mention the GOG store for such events.