Its true on Linux. It wasn't used because it wasn't free software and you had to pay to use it, it was a default propietary UNIX GUI on commercial UNIX computers. You're assuming Motif+CDE has always been open source and used on Linux, it became free software too late, GNOME and KDE became better DE, were free, it wasn't as good as those alternatives by that time and nobody had any incentive to switch to CDE (desktop Linux didn’t even use it aside from a blip that RH did). Motif was propietary software until 2012 and subject to royalty fees, the same could be said about OSS for linux audio, which they abandoned for ALSA since it wasn't proprietary (OSS went from free to proprietary so it was abandoned, by the time it was free again nobody wanted to use it), except nobody used CDE or motif on linux aside from RH for a short time because it wasn't free software.