Probable reasons:
1) Semi protection against cleave that she cannot block. Putting her in the back mid row can still let her tank 2 rows of standard frontal attacks while the 3 other ninjas line top or bottom.
2) This doesnt sacrifice a p1 initiator and give up R2. You could do the same with 2 ninjas in the front column and her in p2, but this formation is cleavable if you're facing chojuro teams.
3) Chase acu still works, and hinata isnt that great a tank when she doesnt have the protection of her mystery.