Why would people build native mobile applications for Android/iOS using Java/Objective-C when they could use Xamarin/C# for all platforms if license cost isn't an issue?
Well, for one, if someone doesn't knows C#...
Plus, Java/Android Studio and Obj-C/Swift/XCode are THE native tools for each platform supported by the manufacturer of the platform. Is someone knows Java and likes it, there is no reason he/she would use C# on Visual Studio!
There are many reasons to choose particular tools - some prefer Java, some prefer swift, some prefer C# or JS, some people are happy to spend longer developing their apps as they already have the skills sets and like the tools.
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