Snapchat: Stories are not autoplay. You touch and see the story you're interested in and the view closes. You have to scroll and open another story.
Instagram: Stories are autoplay. You click once and it's a slideshow. So you can assume metric here are orders of magnitude higher than what it should be.
Snapchat: Story introduced a new revenue source without affecting bottomline.
Instagram: Stories at the top of the app is a very bad user experience. If an users spends more time in story, he spends less time in scrolling instagram feed. (Nobody in Instagram wants to talk about that).
Snapchat: People post whenever they feel like posting. Instagram: People post when they feel something is of very high quality.
Snapchat: Doesn't want to grow the number of users as it doesn't matter to company's revenue. FB makes 75-80% of revenue with just it's 10-20% of users which snapchat already own.