Lack of a tagged stable/release version with libtorrent 2.0 for one.
also: is that really such a dealbreaker?
Probably not a dealbreaker for most but it might be hindering Bittorrent v2 adoption.
Looking at the downloads (I'm using brew) it seems that not many people uses libtorrent2… (https://sourceforge.net/projects/qbittorrent/files/qbittorre...)