If he announced his VP choice now, that would give the LAAP-dog media all the time in the world to attack that person before the convention with the desire to make them toxic.
It would take all the oxygen out of the room and make the VP the focus of the news cycles instead of President Trump and his campaign message.
It would take Biden off of the front pages and effectively put him back in the basement at the time when he's most vulnerable.
It's best for Trump to be aloof about this and keep stringing the media along. That way he becomes the news, not his running mate. Trump can make his message about him versus Biden, not Trump versus his VP nominee.
-PJ