Cost. Specifically cost per Watt over time.
A typical AA alkaline battery is a 1.5 Volt cell that can safely discharge 0.5 Amps and a good one can do that for about 4.5 hours before it is dead. That comes to 0.75 Watts for 4.5 hours. Since even the smallest PV consumes 4 Watts that means you'd need 6 batteries to give a weak vape and keep the batteries safe. Hold 6 AA's in your hand and imagine the size of the PV that would hold those.
A Li-ion is much like a Lithium battery, but the Li-ion can be recharged and it is called a secondary battery where a Lithium cannot be recharged and is called a primary battery. A Li-ion, like that commonly used in PV's, has a nomial voltage of 3.7 Volts and even the cheap, horrible, ones that are not usually recommended for PV's can safely discharge 0.7 amps and do so for a bit over 3 hours. (Note that a non-rechargable, primary, Lithium battery's safe discharge current is usually lower and they tend to not have safety circuits, nor safe chemistry in them either, so using a primary Lithium battery in a PV is much like playing russian roulette. It's not a matter of if you'll get hurt, just a matter of when.) So a cheap Li-ion that is not well suited for vaping can still produce 2.5 Watts in a safe manner and go through hundreds of recharge cycles without any issues. Much closer to the lower end of the vaping power scale of 4 Watts, and all we have to do is go to a better and slightly more expensive battery to get that power level to at least 4 Watts. (Just a bit more to go all the way to 15 Watts if one is so inclined.)
So we'll up the safety and the cost and use a good high drain, suitable for vaping, Li-ion that can be charged and recharged hundreds of times. Let's say that it costs $20 to buy this one battery and the charger that can be recharged 500 times. This comes to about $0.04 of material cost per each 3 hours of vaping.
Compare to a pack of 6AA's that last for 4 hours if giving a vapor so weak that most couldn't even exhale a visible vapor and would last less time if there was a safe way to up the power.
A month of vaping later, you've consumed $1.20 of the life of your rechargable Li-ion or gone through 30 6-packs of AA's. Which would be better?
(Note the examples are based in electronic theory and practice. C ratings and discharge curves have been glossed over for simplification. The price of $20 assumes an average charger and a fairly good 18650 Li-ion type secondary cell. Use only the manufacturer's recommended batteries in any PV you use. The examples are simplifications to answer the question of "why?" and not an instructional tool for DC electrical theory. All questions, comments, and suggestions on how to make this answer better can be dropped into the Middleofnowhere Indiana where they will be found by me and responded to in a timely manner.)
