Most of the comments here already talk about UX or other standard 'issues' with Snapchat, so I wanted to raise a different one—

On Snapchat, there isn't much to do once you're done seeing your friends' snaps.

Instagram has an amazing discoverability feature, plus, you can go around visiting other people's profiles as long as they are public. Ditto on FB, where there is an endless list of things to do and see and explore without interacting with anyone or letting them know of your actions.

On Snapchat, there is nothing of that sort. Snaps are temporary, Stories only stay for 24 hrs, and you can't see anyone's content without them getting a notification (last I checked).

Sure, this prevents 'stalking', but that comes at the cost of engagement.