Ubuntu being debian based is a huge reason for me. Apt is both more familiar and easier to use than yum/dnf, at least for me, but I'm sure for many others as well since most of the time, the first distro they try out is some form of Ubuntu.
* dnf's basic commands are all obvious: install, search, update and info - unlike the seperation between apt-get and apt-cache
* dnf automatically fetches and updates the repo cache
* dnf can rollback entire transactins
* dnf supports delta-upgrades, which speed up things a lot on slower connections
* dnf makes unattended upgrades easy (just add -y), apt may still ask questions even with DEBIAN_FRONTEND=noninteractive
The new apt tool improves on some of these issues, but dnf is still a much better tool in my opinion.
In the meantime, apt-get has been there (and "good enough") from before yum even existed. I really don't like re-learning for the sake of it; the less I need to know to run a box, the better.
People are used to apt. Switching to Fedora means learning an entire set of commands. While dnf's feature set is better in a lot of ways, that's just too much to ask when apt is there and it works for the majority of users, newcomers to Linux or otherwise.