Most of the pros stuck with 1.6 instead of moving to Source (and most existing tournaments stuck on 1.6 - although notably CGS went with CS:S). In Source the movement was clunky, the sound system was awful, and the shooting & spray felt bad. The overall physics was just very strange, it felt like you were walking in honey - although did get better (a long time later).
Almost a year before CSGO official launch there was a showmatch for it and it looked awful. A bunch of small things were improved by the launch, but it was not good at all (look back at 2012 CS:GO footage and you'll be shocked). Sound was inaccurate and inconsistent, molotovs and smokes were glitchy, wallbangs didn't work in a sane manner ... it was pretty atrocious. That said, 1.6 was basically ancient and Valve (who didn't develop the game) pushed for CS:GO to be used in competitions, so it was used. Over time it was increasingly improved and there was an influx of existing (1.6 & CS:S) pro players towards the end of 2012 and early 2013; and an explosion towards the end of 2013. CS:GO came out in August 2012, but only crossed over the 100k concurrent player mark (which is the all-time CS:Source record) in ~ December 2013.
Skin betting became rampant on CS:GO after the August 2013 Arms Deal update, with sites like CSGOLounge having absolutely huge overall parimutuel pots on basically ever game. This pushed up the popularity of the game - even a small local tournament could have 10k viewers if the game was listed on CSGOLounge [and the bigger matches had millions on the line].
Eventually, years later, CS:GO was improved to the point where it could stand alone without propping up by the skin gambling - and since then it's grown much further; but in reality both CSGO and CS:S had very similar and buggy first few years.