It's because he can only do it once, and it's a bit of a nuclear option. If he goes too soon and doesn't have enough evidence of systemic bias and gets knocked back then she isn't getting removed. He needs to wait until she's made enough factual and indisputable errors of law (not just generous interpretations but black and white errors), that the higher court has no other recourse but to remove her.
So far she has made two big errors which got reversed. A third would probably do the trick in proving bias. The problem is that Smith has to appeal for her to reverse her own decisions first before going up the chain. This is why she has suddenly done a 180 after massive delay when ruling the Defence could have access to sensitive documents. That would have definitely been reversed on appeal for national security reasons and then that would have been her third strike.
Basically, she can keep delaying and making errors, as long as she reverses them before it goes to appeal.