Because Debian has already done much work on integrating AppArmor [1].
And Debian based distro's like Ubuntu, Tails, and Subgraph also work on AppArmor so choosing AppArmor over SELinux means overall less work for the Debian community.
SELinux is more secure, flexible and comprehensive, so Debian should adopt it by default...
Anyway if AppArmor gets selected by default, I hope I can switch somewhat easily to SELinux if I want to.
And Debian based distro's like Ubuntu, Tails, and Subgraph also work on AppArmor so choosing AppArmor over SELinux means overall less work for the Debian community.
[1]: https://wiki.debian.org/AppArmor/Progress