Steam has made quite interesting progress and also serves as platform for games being released on different OS. In addition, producers don't have to worry about DRM or delivering updates, which is a point you really should take into consideration to understand their reasons.
Gog, on the other side, has upgraded their services and is offering more and more modern games instead old ones before. With GOG Galaxy they did one step forward to offer chat and update delivering quite as Steam does.
Which I do not take into consideration are services from producer companys themselve as Uplay, Origin, Battlenet aso., as they only try to bind customers toward single developers over devasting DRM pressure.
Anyhow, I'd vote for releasing on other secure platforms to have more ... competition, otherwise I rate Steam getting distribution monopoly once, which can't be good for anybody.