According to the Wrestling Observer Newsletter, one of the main reasons that Kevin Owens was turned heel is because of the uncertainty surrounding the injury to Daniel Bryan.
Originally, Owens was expected to be the #2 babyface on Smackdown behind Roman Reigns in a “guy next door” type character, who ate junk food and shared much of the same problems that the general audience would be dealing with. This is why they also gave him the Stunner as a babyface finisher.
However, with Bryan’s situation unclear, his rematch against Kofi Kingston at Money in the Bank is likely off and so Owens was needed to turn heel to fill that position.
The only other option for WWE was to use someone like Randy Orton or move over Drew McIntyre or Baron Corbin. A few days after WrestleMania, the decision was made to simply turn Owens.