It wasn’t just CS, Team Fortress had a huge player base. Natural-Selection as well. There were like 7 or 8 mods for HL1 that were “on-par” with the quality of HL1, which says something about Hammer Editor and the toolkit they had at the time. GTkRadiant is great but Hammer Editor (formerly WorldCraft) was excellent.

The popularity of the mods, CS included, is what drove Steam to become the store that it is.