Displaying an unwillingness to use your weapon
If you listen to the account of Sarah McKinley's shooting of home invader / would be rapist Justin Martin, she said something to the effect that when he broke open the door and stepped inside, he looked at her and when she didn't shoot, he took that to mean that she wasn't going to shoot and then he came for her.
I don't remember her exact verbiage, but she relayed that she beleived that he made a decision that she wasn't going to shoot. He forcefully shoved a couch out of the the way then Sarah pulled the trigger and shot him in the head with her 12ga.
I say Martin was a would be rapist because that's what Sarah believed and also because he had been through the house and stolen her deceased husband's prescription drugs already. If he were merely after drugs, he had already gotten what he wanted or could get what he wanted at some future date when she left on errands because she didn't lock her doors normally. But he chose to break into her house while she was there. She thought it was to rape her and I think it makes sense.
Anyway, there are some predators out there who will react to any hesitation by immediatley launching an attack.
If you've done the warning shot you've just reduced your available ammunition by one round and it's also likely that you've allowed them to get somewhat closer to you.