Only real reason would be the ability for the GTK dark theme to apply when using the "CALIBRE_USE_SYSTEM_THEME=1 calibre" command along with qt5ct. Looks good in my VM that I tested it in using the distro package in Pop OS:
https://i.imgur.com/eWHj6Zy.png
As far as i'm aware there is no way to make theming work inside of the official version due to it being a container like install right? (no way to export the theme into a location it can be used right?)
Currently trying to decide between having the newest version vs having it look like the rest of the system, so just wanted to see if it was even possible to go back without there being any issues first :P