Bug#727708: I'd like to voice my opinion
I'd like to request that upstart be chosen over systemd mainly because there's already a large availability of deb packages that support init mainly due to ubuntu. Ubuntu acts as a gateway distro to the debian universe, and is a basis upon which numerous other distros are based as well.
As such, a lot of packages are developed for ubuntu instead of debian. Making upstart the default init package allows for compatibility with the majority of these ubuntu specific packages.
Furthermore, I know upstart is capable of maintaining backward compatibility with systemvinit scripts as debian implements them currently.