an't think why, the Q400 is cheaper to acquire and cheaper to operate than the ATR. Equally crappy quality though.
Think you should check your sources, Q400 is not cheaper than the ATR to operate. Yes it's cheaper to buy but the ATR72 comes in around the same price as the Q300 to operate.
Like I said the ATR's are here until a bigger (90 seat) bird is available. Q400 was written off a couple years back when Mt Cook was looking at a fleet replacement.