I have about 2k hours racked up on 1.6.  I actually jump in between 1.6 and source occasionally.  I would say 1.6 is more popular than source because it doesn't take a top of the line computer to run it.  1.6 is about 9 years old (not including the mods before it) and anyone can run it with extremely good reg.  On source, you have to have a decent computer to run it.

Not to mention, when source first came out, the mod did not run well at all.  Hitboxes were huge and lagged behind the player model.  It was never as smooth as 1.6, etc.  I actually used to own a css community when it first came out but went back to 1.6 shortly after because I hated everything about source.

If you ever notice, source is finally starting to climb over 1.6.  The only reason for that was the release of css beta and the whole re-worked mod.  That brought a ton of 1.6 players to source.

I would say the more die-hard fans of cs stay with 1.6 while the new-comers generally start at source.  On 1.6 games, it's rare to not see a 5 digit or lower steam id in the server.  On source, usually I have the lowest steam id (7 digit).