I think he sees this case is much more glamorous- I'm sure he is well aware of how the left will fawn all over him should he manage to bring an indictment against Rove (who is most obviously the real target, not Libby).
There's no question that Karl Rove is the target, the left views Rove as the greatest threat to liberalism.
On the other hand, almost nobody even knew who Scooter Libby was until a few weeks ago.