Ben Finney comes closest to the most probable explanation for the lack of developer interest, I think. Canonical chose to do everything in their own walled garden (Launchpad) with their own technologies (Bazaar-NG) and expected people to play along.
It's one thing to have various technology decisions influencing any project that Canonical is involved in, even though this will deter outsiders by itself; it's another to see Canonical wanting to exercise control over the products of such development as well.
You can certainly lean on the community to do the hard work if they benefit as much from it as you do, but as soon as you ask them to benefit less (and here I ignore the excuses about Canonical's unique stewardship role and corresponding privilege - on whom does the hard work of quality assurance and other tedious stewardship matters fall, exactly, if not the community in many cases?) then they will exercise their rights as volunteers and indulge some other projects instead.