Everyone you know at what age? Teenagers continue to use the two products for very distinct uses. Snapchat still has a ton of features (groupchats, geofences, geofilters (+ custom user-made ones), etc.) and is probably the #1 messaging app at most high schools. Instagram stories haven't caught on nearly as much (some people use them, but very differently from Snapchat stories - 1 high quality selfie/arts-y pic that fits the normal Instagram style every day vs. an actual "story" of their day). Asking why someone wouldn't use both is like asking why someone would use both Twitter and Facebook when Facebook can do everything Twitter can.