Because that's how it works. From indications the girl in question did not attend the police station on the night but spoke to police at the scene before going in to make a formal statement the following evening. She would not have been DNA tested until then. Similarly, Brett Stewart would not have been arrested by police until hours after the alledged attack and may have not provided a DNA testing that night either, especially he deferred making a statement.
You would bother a day later because it makes sense legally and even though chances are any DNA remains would be gone, there is still the slight chance that they would remain and tests would be taken out regardless.