Play Counter Strike 1.6, with full multiplayer, in the browser

Source wasn't well received by a portion of the community. The community bisected for a while with some folks staying on 1.6. I don't recall everything behind the 20+ year old drama but it was a thing. Source eventually won out and the rest is history.

source engine is different from goldsrc, and the "feel" of goldsrc goes back to q2 (and some idiosyncrasies from q1). so if you spent several years playing from beta days, or for some people transitioned into cs from even more years of q2 or q1, then source would just be an entirely new experience. there's nothing valve could've done to preserve the right feeling, which is fine. old timers continued playing goldsrc until full blown grown up took over, where's new timers were formed by the conventions of source and further.

IIRC Hitboxes were different in the early days which kept many folks on 1.6

Don't think Source ever won out, Global Offensive was the game to dethrone 1.6. At least in our "semi-professional" neck of the woods CS:S adoption was 30% at best. A clear majority were still playing 1.6 even when Global Offensive came out, and even then it took a year or two of updates to make that game good enough to start convincing significant portions of people to switch over. For example in 2011 Electronic Sports World Cup just before Global Offensive came out 1.6 still had a significantly larger price pool than CS:S. Funnily it was also the first and last ESWC to even have a CS:S tournament, even though the game had been out for 7 years at that point.

Cool. Thanks for the history. I fell off CS except for occasional casual sundays and dove head first into MMO land around that time so it's all rather fuzzy. I kinda forgot Global Offensive was it's own release.