How can I vote from other platforms to Xbox on Twitch?

I'm streaming from the Xbox to Twitch. I can if I do not stream the voices of PS4 or pc when I'm streaming Fortnite you just hear me and that annoys me and my viewers does anyone know how to fix that?

That does not work on Xbox. You need a recording device from Elgato where everything is recorded.

But why does that work on Ps4?

Because Ps4 and Xbox are different consoles, and Xbox provides more privacy when people do not want you to hear videos or streams without asking.