Honestly, as a platform, GoG is objectively better. It lacks innate DRM, has all proper features of a digital storefront (friends, auto-updates, cart, etc) and matches, in not beats, steams sales.
There are 2 main reasons it hasnt topped steam: It's focus more on older titles, and people wanting all their games under one roof.
The first reason is their own choice, while the second is something ALL digital storefronts have to contend with due to Steam's age.
As for the goodwill thing: At first steam was highly curated and exclusive which ensured that customers knew that if it was on there then it met a certain standard (i.e. it worked and wasn't illegal). This built up a massive amount of faith in the platform for both game devs, who got free publicity by just being on there, and Customers who knew that anything they bought was checked to ensure that it worked as intended.
After the advent of Greenlight, though, the overall quality dropped and it became harder and harder for new games to be found under piles of DLC and Asset flips. A problem that has only gotten worse since then.