#4 To present him as a babyface
Bray Wyatt was a face before Matt Hardy took a break from wrestling. They were the RAW tag team champions but were forced to drop the title due to Hardy's physical condition.
Wyatt was presented as a face only because he was in a tag team with the Woken Matt Hardy. The character of Bray was always viewed as a villain who got more and more ineffective over time.
Bray shed his old cult leader persona to become a babyface. This might change the perception of his character, and help him establish a connection with the WWE Universe.
#3 To cut any ties with Matt Hardy
Bray Wyatt was last seen as Matt Hardy's ally. Since then, Matt Hardy has returned and reformed The Hardy Boys, going on to win the SmackDown tag titles. Also, Matt is now using all his gimmicks at the same time which makes it difficult for Bray to comply.
Bray made his return on the RAW brand, giving him the chance to carve a separate path. By giving him a different gimmick, WWE wants to cut down all the ties between him and Matt Hardy. Now, Bray Wyatt is a family-friendly superstar, who is intended to target the younger demographic of the WWE Universe.