Because his blood was on it, too, and perhaps he didn't realize that Danielle had bled on it? Posters here keep emphasizing what a small spot it was (though there shouldn't be any spot at all), so maybe he thought the stains were all his.
Puh-leeze! If he KILLED her while wearing that jacket (or the jacket was somewhere blood could have splattered onto it) and then noticed there was BLOOD on it, wouldn't such a careful killer simply ASSUME it was HER blood or at the very least a combination of his and hers? Remember, this is a guy who the prosecution claims made gargantuan efforts to eliminate every bit of trace evidence he could -- I can't imagine why he would think it was his blood and HIS ONLY if there were ANY POSSIBLE way her blood could have gotten on it during the commission of a murder. If, on the other hand, Danielle DID get those few drops on it during the Girl-Scout cookie session or while playing in the motor home unbeknownst to him, Westerfield would have NO REASON to suspect it was anyone's blood other than his own (or perhaps an animal's if the guy ever went hunting or fishing, forgive me for not doing research on his outdoor sporting actvities!) and would naturally have no qualms about taking it to be cleaned. Just my opinion, of course....