Protection of your success is paramount. The moment you stick out your head a bigger, fiercer, more experienced, deep pocket players comes around and clones you. Right? Unfair!
Your question is a good one. In my experience there are several things that make it hard to duplicate success even if the app itself is easy to clone. Four come to my mind immediately. On different abstraction levels and certainly interconnected.
1. Protection by IP
Intellectual property. Hard to get. But pretty solid. Often not applicable. Beware of lawsuits against big guns.
2. Protection by user
In essence a speed advantage. Consider switching costs. https://www.investopedia.com/terms/s/switchingcosts.asp
Luring users away from the original can be freakishly expensive
3. Protection by talent
I would always bet on the better team. Often companies get acquired just for talent. A well oiled machine can out-innovate, out-speed, outwit even the biggest opponents.
4. Protection by insight
Markets need to be played constantly. Adjust. Be nimble. Just copying one already working angle does not mean that it will work for long. A team with insight into the dynamics of the apps surrounding can react properly and over time even intuitively.
5. Protection by feedback
Not a very clear distinction to the other points here - its more a practical example of speed (2) and insight (4) - but i want to elevate one point. If you get ahead on the user base you can collect feedback better (quality and quantity) which allows you to react more precisely - giving you the edge.