To be honest I don't really know why other than that perhaps a lot of the people that play Counterstrike1.6 still play on low end computers.

I've found that they both play almost exactly the same although there are some things that make CS1.6 a little more annoying I guess such as having to buy primary ammo(which im sure most people have binds to do stuff like that).  Also it seems to be harder to find some actual games in 1.6 as in there a lot more fake servers i come across as well as just not "normal" games that I would enjoy.(sorta like if you go to play Warcraft 3 and all you find is people playing Dota).  This problem can eventually be solved by favoriting servers so you dont have to search.

But yeah I dont really know why 1.6 is still as popular as it is today.
Stats from steam at time of this post:
Current Players Peak Today Game

13,085 76,059 Counter-Strike: Source
12,217 57,801 Call of Duty Black Ops - Multiplayer
12,116 76,021 Counter-Strike
8,696 22,168 Team Fortress 2
8,382 24,347 Sid Meier's Civilization V
7,327 18,917 Left 4 Dead 2
7,299 40,641 Call of Duty: Modern Warfare 2 - Multiplayer
4,045 25,786 Football Manager 2011
3,922 12,566 Fallout New Vegas
3,843 16,662 Call of Duty Black Ops

It wasnt until the last few months that CS:S finally has been above CS1.6  and i think it's because of the update to CS:S that came out this summer I think that added achievements and revenge kills etc.

I like CS:S better(if i have a machine that can play it) if i'm on my laptop I will play 1.6