You have to understand this:
Upstart is modern, it works and it is well tested (does Systemd have a decent test suite now?).
Ubuntu does not need to switch to Systemd right now.
I do agree that having one standard init system everywhere would be great for many reasons. And Canonical would probably be happy to spend man power somewhere else than Upstart.
I think they may reconsider this after 14.04.
But such a change is not trivial. Especially with such a fast moving target as Systemd.