Why unmanned exploration is preferred to manned exploration at the moment?

It would be less expensive for a start. The risks in manned exploration is great, a lot of things could go horribly wrong and lives lost. There is also the question of what If we landed on a planet that was inhabited by an alien race? What would they do with the astronaughts? It would be better to send an unmanned craft to planets capable of supporting life in case we find it.