Because Raspbian is the general purpose operating system that includes all of the packages you need and currently (because I'm waiting for the postie to deliver some new SDCards) Ubuntu is an unknown beast.
Raspbian is also runs on A,B,B+ & A+ so everything you do on it is backwards compatible for the folks who don't have (and don't desire) the new hardware.
Note: Any requirement to use a crystal ball or mind reading will result in me ignoring your question.
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