The reason they did this is because they had a huge hassle getting everyone to move over back when they moved CS to the source engine.
You missed a generation there. CS 1.6 -> CS:Source -> CS:GO.
This isn’t in response to people not moving over to the Source engine, there wasn’t much issue with that - although 1.6 still lingered on for a long time and people did complain, many people were excited by the Source engine and all the new physics it introduced, so people did buy Half Life 2 (with CS:S) even if they preferred to play CS 1.6. However with CS:GO it was different, there were no significant new features except the hat-ification and skins along the lines of Team Fortress 2, also CS:GO was just a standalone game rather than bundled. So many people did not move to CS:GO.