AAA publishers are probably still with the "it's DRM-Free = more pirated" mentality, while fully knowing their games on Steam get pirated at release regardless of GOG.
That said (based on my opinion):
- Some of them would have to maintain up to 6 different versions (Windows/Mac/Linux), 3 per distributor.
- Releasing patches on GOG is somewhat slower as they have to work around Steam-Only features such as the Workshop.
- For Linux, devs have been provided a framework which basically eliminates many dependency issues across the different distros (This may or may not the reason why so many Linux versions are missing from recent releases).
- Greenlight/Beta/Early Access features. Beta may be passable but the other 2 are more complicated than simply adding the feature in Galaxy, feels out of place considering how many GOG users refuse to use a client and devs would get very little feedback in return.
- Self Publishing, GOG maintains a curated library unlike the Jungle that Steam is but also has its drawbacks.
I for one don't wish for GOG to become a DRM-Free clone of Steam but wouldn't mind a little more AAA releases now and then (Yes Oblivion I'm still waiting for you).