DubConqueror: Two reasons:
The store started off as Good Old Games, with the intention of releasing old games that were hard to get and tweaking them so they would run on modern systems.
When the store expanded into releasing new games as well (which GOG needed to do for continuous income), many publishers of 'new-ish and AAA level' games, are afraid to put their games on a DRM-free store as they (falsely, I think) believe that putting DRM on their games will get them more sales (they get pirated otherwise, is their reasoning) and they only release their older titles here, that already had many sales but will get a second life because of people wanting to get them without DRM. Proving release without DRM is a selling point, therefore making their first argument moot, moreso because games get pirated anyway, despite DRM.
people will pirate games, if they have DRM or not, but why should i support developers who despise GOG and its users mere existance they dnt deserve my money they can bugger offf