It's still around because some people refuse to leave it, even if the new version was much better. And they're so vocal about it at the same time.
It's the same thing that happened with all the attempts at combat improvements. The people at the top threw fits because their advantage would have gone away. Never mind that the game as a whole would have been improved, but THEY would no longer be at the top. It's the same with the people using the flaws in the old client to their advantage.
No, not everyone using the old client is like that, but the ones who have the most to loose from it being dropped react the worst to it, so the devs have to concentrate on two clients instead of one. If the devs (programmers?) were allowed to work on just the new client, it would (probably) be working better by now.
And before anyone says they can just keep the old client, they've stated before that the only way to fix the problems that the exploiters use, and just plain bugs that people think are features, would be to rewrite the entire client anyway. In other words, a new client.
Things like speedhacking, which I see so many complaints about, are due to flaws in the old client. They could conceivably be fixed, but the effort to do so would be comparable to just making a new client anyway.