BrokenBull: Perhaps GOG has good reason to reject certain games. Do they publicly state the reasons for rejecting games?
They don't, and even the developers usually get a generic answer such as the one mentioned in the OP.
I'm pretty sure I know the answer though: GOG wants to remain a curated store which offers some amount of exposition to games that enter it. If you noticed, every new release is featured in a newspost which seems to be written by GOG people, and they seem to all be featured on the front page for some time. GOG is also commited to having a lot of goodies that come with each game, and to make them run as much as possible on future OS's. This is of course, only feasible with a trickle of games entering the store.
Steam used to be more like that (which is why making it on Steam was huge for indie games, the advertising they got almost automatically meant success). But now that Valve have opened the floodgate, tons of games go straight to the catalog without ever getting a frontpage spot.
This does pose the problem that if you want to have all your games at the same place, the Steam client will win everytime. I wanted to transition to GOG Galaxy, until I realized that (besides the bugs) I could only have a fraction of the games that I buy on Steam on there. Also, very few bundles offer GOG keys so my main method of trying out indie games doesn't work with them.
Maybe GOG should consider having two very separate release channels and a distinct split in their catalog. Premium, manually curated games would be handled as usual, and games that didn't make it through the selection process for some reason would be let in through the backdoor without publicity, quality control, emphasis on goodies or commitment to make them run on future OS's. The only policy would be that they must be DRM-free. To prevent drowning the featured games in niche/crap games, the secondary catalog could even be featured on a separate website/store, which I suggest naming BOG.com :P . Seriously, though, the key is that the Galaxy client would handle both, allowing you the best of both worlds: a nice curated store on the one hand, and a sea of games to explore on the other, and the ability to have all your games at the same place including very niche ones. All that DRM-free.