It's seems like the XAML they went with is actually the better option -- it's close enough to other markup languages that any developer can pick it up with minimal effort, but not exactly the same so they can make any changes they want without hurting anyone else's compatibility or the larger ecosystem.
And (totally subjectively) XAML is actually nicer for developers and users than HTML+CSS+JS in some areas, if your willing to invest just a tiny amount of time to learn it.
--
Plus, Microsoft's UWP also lets you use a web browser too, if you prefer. If you feel your markup is better in HTML+CSS+SVG, Microsoft already supports that as well - https://docs.microsoft.com/en-us/windows/uwp/get-started/cre...