1. Ubuntu invested very heavily into making Linux friendly to a whole generation of makers when nobody else was. Ubuntu is most familiar to them. Canonical will benefit from that investment for the foreseeable future.
2. Ubuntu benefits from Debian's debootstrap which makes porting to a new architecture substantially easier.
debootstrap (or mmdebstrap) is just for installing the already existing binary packages. The bootstrap process for bringing up a new port is a lot more work.
The reason why Ubuntu is probably that they are a commercial vendor so you can make contracts with them, while the likes of Debian just work on what they care about when they have time.