A really good question!
I've lanched an App and "it has been cloned".
What makes and app successful depends on many things...
- If you are the first launching it.
- Your marketing strategy to reach the correct people
- How the client benefits of your app, is it relevant? Useful? interesting?
- Is your app user friendly and easy to share?
- And many more...

Also there are a Lot of things that the competition cannot copy, and in those things is what you have to focus.

I hope this help at least a Little.