As a user of both Instagram and Snapchat, this doesn't surprise me. The UX for the Instagram stories functionality is pleasantly at the top of the normal UI, but Snapchat's UX is really quite hostile, and even though I'm very familiar with it, it continue to be unpleasant to use.

Add in that Instagram's typical user base is much more broad (Snapchat tends to be most popular with young tweens, whereas Instagram's user base is just about everybody), and you have one actively hostile interface in an already small market against a much more friendly interface in an almost unrestricted market.