There are a few prevailing theories.
Since Instagram launched Stories in August 2016, the Facebook-owned company has been steadily increasing in popularity, mostly at the expense of Snapchat. In early 2017, Instagram Stories overtook Snapchat in terms of audience size.
One of the early differentiators between Instagram and Snapchat was Instagram’s heavy reliance on algorithmic ranking. In June 2016, the Instagram feed moved to an algorithmic model (it was previously chronological) and the order of Instagram stories at the top of the app is based on engagement data from Instagram and Facebook.
Original source: https://socialmediaweek.org/