A basic Google search leads me to this article [0].

> On March 27, 2026, Start9 CEO Matt Hill hosted a private unveiling of StartOS 0.4.0, the next major version of the operating system that powers the Start9 Server One. During that same session, Hill also gave viewers a first look at StartWrt, the router’s dedicated operating system. StartWrt is Start9’s fork of OpenWrt with a modern GUI that reimagines the router experience from first principles. The interface is sleek, modern, and a clear departure from the technical admin panels that define most open source router software today.

> Where OpenWrt’s default LuCI interface is functional but technical, StartWrt presented a clean, modern interface designed for users who have never configured a VLAN or written a firewall rule.

When you consider the circumstances a fork is the only thing here that makes sense. You can't just open a pull request to OpenWRT where you are like "Here is our purpose built simplified GUI we designed for our router, please merge."

[0] https://www.solosatoshi.com/start9-announces-fully-open-sour...