Because you'll be asking him to play a role that has a higher risk of injury it would make sense for it to be a cheaper talent. Paying for Austin and him not having a significant role in the offense wouldn't be very smart.
His ceiling as a WR has been maximized, at least if you took a rookie with a lower floor but a ceiling that could work towards being a contributor you aren't paying millions for him just to return punts.
Just makes more sense to draft for that role.