a. The one which runs on Ubuntu by default and which is probably used on 90%+ installations of desktop Linux is GTK which is utterly a major nightmare to develop GUIs with.
b. The Qt library which powers KDE is simply not used all that widely. The flagship desktop environment using Qt is KDE which is downright ugly in its default installation.
2. Once problem 1 is solved, people should realize that a usable desktop is not just the Window manager and 3 apps which are blessed by the desktop environment team. It is 100s of thousands of hours of using by many users. It needs stability for other apps to develop around this ecosystem, both Mac and Windows have had a rocksolid GUI library powering their ecosystems for decade+ period and apps develop around them.
Without stability and a new shitty GUI library every 2 years, Elementary OS or not, the desktop environment situation on Linux will continue to be shit.
Much more damning is that RedHat whose entire business model was around making an enterprise ready Linux (server and Desktop) could not build a decent GUI toolkit.