Q3 requires a relatively large amount of time to become good at the game. The thing is that someone who is good at the game cannot be killed by someone who is new or merely average at the game. I was "above average" at Quake, and generally smashed people who were below my skill level to pieces. Unfortunately, the same thing applied to me whenever I went up against anyone who was better than me.
Newer games are designed to give newbies, or in general, players who are not as good, a much greater chance of killing the best players. This keeps the game fun since they are not getting pwned every time they spawn, which keeps them playing. Games like Q3/SC2 are generally only populated by the uber-hardcore players, which leaves little or no room anyone else.
And there's quite a few people who still play QuakeLive, so check that out if you're keen for some Quake.