Screen mirroring from a Windows computer is usually more of a Windows issue rather than a Roku problem. Windows has very specific hardware requirements for it to work. I have found the screen mirroring works fine from Android devices, and it's almost impossible from Apple devices. There's just no "standard" for mirroring, so everyone does it differently. If I want my PC to display on the screen, I just connect it via HDMI or VGA connections. Mirroring is just too much of a pain.
Except that Miracast is a standard, built on standards, and not a new one:https://en.wikipedia.org/wiki/Miracast
"Miracast is a standard for wireless connections from devices (such as laptops, tablets, or smartphones) to displays (such as TVs, monitors or projectors), introduced in 2012 by the Wi-Fi Alliance. It can roughly be described as "HDMI over Wi-Fi", replacing the cable from the device to the display.
The Wi-Fi Alliance maintains a list of certified devices, which numbered over 6,700 as of 9 March 2017"
The fact is, two major device OS makers either never opted in (Apple), or later opted out in favor of their own proprietary protocol(s) (Google) - and both are members of the WiFi Alliance.https://www.wi-fi.org/membership/member-companies
The standard exists, but so do non-compliance agendas.