amok: it is simple maths also (and I used made up numbers....)
Assume you make a game which 0.5% of the population is interested in.
Releasing the game on for example gOg, with about 200.000 users means a potential 1000 sales
Releasing the game on Steam witt 7.000.000 users means a potential of 35000 sales.
You may then ask if it is worth for you the hassle to chase down the extra 1000 sales (both in terms of monies and time)
But, this seems to assume that visibility on Steam and other platforms is more or less equal... with hundreds of indie titles being greenlit every month adding to the thousands already there, I don't see how that can be the case. A new release on GOG might get the - at least momentary - attention of the full active userbase; the chances of a release on Steam doing the same appear minimal IMHO, unless it's a title with e.g. lots of pre-release hype going for it.
Glasswolf: He forgot to say that Gog also only allows a) good b) old c) games on the site. Probably why he didn't get on.
Umm, GOG stopped focusing on just "old" games a couple of years ago... note how the current promo is all games from the last few years.