If you look at the other
Noitamina series, you'll see that 11-12 episodes is pretty much the norm. Honey & Clover I and Nodame Cantabile were the only series to have over 20 episodes; and H&C's sequel was only 12.
I'd agree that NC's strong ratings performance in all its incarnations might have suggested a longer run; I guess Fuji's just a bit more risk-averse, but then it's their money.