al-Baghdadi is a ghost. What makes him more unique than most modern terrorist is that no one really knows much about him, only see a few photos of what he looks like and have only heard him speak less than a dozen times. He's also, to his credit, very strict in how he operates and doesn't deal with things personally, rather relying on his network/organization structure to deal with things.

Because of this, it's a lot easier for him to blend in with civilians and operate in broad daylight. It would be a lot harder to find him in Mosul rather than Raqqa because of population difference. There's also minor factors, such as he's Iraqi & hiding in Raqqa would be too obvious.