Because he's their uniting force. Without him the others aren't as much threat, and he's the one more likely to be such a danger. He was the one to do the worst of the crimes, and he's the one who wants to be Pirate King.

[The one who was caught by Buggy and smiled at his own death just like Roger, in the very same place in the very same way...]

The others are mostly weaklings anyway or could be dealt with by others. Roronoa Zoro is his swordsman, but my second-in-command wants to deal with him. [Nico Robin still concerns him, but that's a huge headache all on its own with his friend & ex-boss: Aokiji, world government politics, and things Smoker knows he doesn't have all the facts about.]