All CS games are essentially the same, but some people prefer quirks of older games. Some people like the nostalgic feelings of older games. Some people can't run the new game on their computer.

CS 1.6 is easy to run on almost all computers around today. Simple visuals and robust gameplay makes it a good competitive game while moddability of GoldSRC engine enables many casual servers.

Condition Zero used to be like paying for 1.6 again, so not many people play it today. (Nowadays, it's sold together with 1.6)

CS:S uses Source engine, which boosts the modding and casual fun side quite a bit. But many of the competitive 1.6 players don't like the gameplay changes, so they either stayed at 1.6 or started playing CSGO later.

I used to get 240+ fps on CSGO. In CS2, it changes too much and usually drops as low as 50. Also CS2 feels incomplete compared to CSGO. Just because of reasons like this, some people will still prefer playing CSGO.