I also wonder why they wouldn't work with upstream in improving the existing GUI (or upstreaming their improvements), instead of putting the burden of a fork upon themselves.

Working with upstream is most convenient for their users, for them, and for the ecosystem as a whole.

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...

The gui of openwrt is not great. It might be better if you already have lots of experience with linux networking and openwrt specific command line configuration. If not it seems like a mess, very vague and overlapping controls without much explanation. DDWRT and Tomato are much better although openwrt might be more powerful without resorting to straight firewall and routing rules through text.