It was using wxWidgets, not GTK directly IIRC
Most people that have never maintained >=Qt5 projects assume free wxWidgets native cross platform OS interface support is less hassle.
Refactoring the application core means their team should have forked the project to audacity-qt rather than inject long term Qt library and license dependency issues into the original package.
Qt on occasion has proven to cause problems with license term changes, GL library bugs on Linux window managers no one ever fixes, and a chain of legacy build issues.
Microsoft people tend to design around entertainment eye candy rather than functional long-term reliability. A interface use-case context pop-up isn't a design choice, but rather just chaotic depending on who is using the seat.
The Audacity 3 interface certainly needed a lot of work, but nuking it for v4 re-write is simply packaging a different program as the original.
Hopefully the remake works out. The odds are not in their favor though... =3
Ah, the dream of "one UI toolkit to rule them all" was very much alive when wxWidgets got popular around the turn of the millennium