This was actually something i never thoght of before. I was watching a review by LZRD WZRD, on Rarity Investigates, and he mentioned how Soarin should know that Rainbow Dash, as the element of Loyalty wouldn't betray them. He also mentioned that she turned them down twice because of her loyalty. Then i realized: wouldn't that implicate her being the culprit further?

I also figured that Soarin' thought that Dash wouldn't have done that, but he has a whole team that does, and as second in command needs to speak for hem. Also, he might have suspected Wind Rider, but if he was WRONG, it might paint him in a negative light, lose him some respect from his team or, worse, implicate that he might be a suspect. He was fairly sure that Dash would come through as she always has, which would also explain why he told her to find evidence.

After posting that i realized what i just posted and thought

"Eureka!"