Most of the reader here will never have heard about Motif.
Back when I started working with X, there were two competing GUI standards: OpenLook and Motif. Free Software (the term Open Source wasn’t invented yet) stuck to the basic Athena Widgets or (later) used Tk. Despite the strict licensing scheme of Motif some projects (e.g. the original Mosaic Browser) went with Motif nevertheless — motivating the Lesstif project.
It could have owned the Unix GUI market, but it wasn’t compatible with free software, so it lost the battle for developer mindshare and fell into obscurity. Qt almost made the same mistake, but they were a bit more liberal in the beginning and as GTK took off, they had to open up even more to stay relevant.
In other words, the decision not to support free software with Motif killed it.