Probably because its origins are as a UWP-first framework which runs natively on platforms that support it. Skia for Linux was likely added on later to allow the original app to run on other platforms.
I think probably because Linux has no true native UI toolkit, and the two that are closest (GTK and Qt) both bring their own sets of challenges that skew the cost:benefit ratio negative, while this isn’t true for the toolkits for other platforms.
I have no involvement in this project however, so this is just speculation.