Definitely, all of us know about powerful JavaScript engine, So why in React Native is used a different engine that name is JavaScriptCore.
The JavaScriptCore does not support some ES6 features like below function:
Array.prototype.flatten
What is benefits of JavaScriptCore to V8? Why the Facebook developers didn't use V8?
V8 does not run on iOS, because Apple does not allow third-party apps to generate code at runtime (a.k.a. "JIT-compiling"), which V8 heavily relies on for performance (*). JavaScriptCore, being made by Apple, is allowed to run (and JIT-compile code) on iOS. Since React Native's purpose is cross-platform development, this is a strong argument.
That said, Array.prototype.flatten is not an ES6 feature. It is currently a "stage 3 proposal", meaning it will probably soon become an official part of JavaScript -- maybe ES2019 ("ES10" in the old naming scheme) or so. Also, it has been renamed to Array.prototype.flat due to web compatibility issues with the name .flatten. JavaScript engines have started to implement it; according to MDN the latest version of Safari/JavaScriptCore already support it, so it's probably only a matter of time until support arrives in React Native too.
(*) There is an ongoing effort to build a version of V8 that avoids all runtime code generation, trading a lot of performance for the ability to run anywhere, but it's not available yet.