Batteries. Why use 9V?
A 400mAh 9V battery will last a year with a 40µA current draw.

Now consider a smoke detector. It is low power analog circuitry, most likely drawing less than the 40µA figure above. If you wanted to power it from a boost converter and AAs, then you'd need a converter with very low idle current.

But... when there is fire, now you need quite a bit of power, and enough volts, to drive the piezo loudspeaker. These need voltage. 9V is louder than 3V. 

So your very low idle current DC-DC converter also needs to output high current if needed.

You also need to be able to measure state of charge accurately on the AAs.

All this will cost more than the difference between 9V and 2AA. And remember, the customer pays for the replacement batteries, not the manufacturer!