The Foundation had originally intended to use Ubuntu and, IIRC, had some portion of Ubuntu 9.0.4 running on some pre-production boards, but never made it available. Ubuntu 9.0.4 is the last distribution to support ARM v6 microprocessors as used on the Pi and the Foundation moved to Debian when the Ubuntu team stated that they would not be supporting ARM v6 beyond 9.0.4. The Ubuntu team is dedicating all of its resources to supporting ARM v7 and beyond due to multi-core features (Coretex) that can be available in those processors, but not in ARM v6 devices.
Raspbian is a perfectly fine distribution for use on the Pi, even for beginners for whom Ubuntu was originally developed. It includes a lightweight GUI/desktop, browsers, IDEs, and other software best suited for the capabilities of the Pi's ARM v6 CPU, FPU, RAM, SD card storage, etc. The current Ubuntu distribution would never be able to run on the Pi anyway, even if ARM v6 were supported. It requires far more resources than the Pi can provide - 1 GHz+ clock speed, 64-bit data and address buses preferred, multi-core preferred, 1 GB+ of RAM, tens of GBs of disk space, etc., and those are x86 specs - ARM specs would be even greater to achieve the same performance.
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!