Their theory is people will learn her in QP before picking her in comp, but in reality they're probably just delaying people learning her in comp by a week :P
They hope that people will test her out in PTR and then adding the delay will get those that won't do PTR to test her in other modes where people won't rage at each other
New heroes have the same affect as hanzo or widow where people immediately freak out and demand they not play it because they're obviously going to be bad