As Don said, for those that want more range they can just get a 14.4 model. A power tank costs £3k but a 14.4 only costs a little over £2k more than a 7.2. The only cases where it makes sense is for those who get a particularly good deal on a used 7.2 or if they don't have the money up front to step up to a 14.4, but those are a small percentage of an already small market. I suppose you could also argue that saving 20kg could be a reason to add one to a 7.2, but even then I think most people would take the extra weight if it means it's placed lower down.
Pricing in U.S. is basically 11K for S and 15.5 for SR. The power tank runs 3K and part of that pricing is tank/plastic swap out. Basis of business is normally to sell more stuff and promote safety in your products. If battery add on was lower and more central it would be more desirable (more sales) as well as safer. It also may add to more charge tank sales as the tank area would be unused.
You're comparing an S to an SR, the price difference is down to more than the battery capacity. Zero don't list the 7.2 S/DS on their site anymore despite them apparently still being available however the FX/FXS do come as 7.2 versions and their pricing is very close to the S/DS.
The power tank's primary purpose and selling point is to increase the range of the 14.4 models, that's what allowed them to brag about having the longest range electric motorcycle, and that is what most power tanks are installed on. I wouldn't be surprised if the cost of designing and manufacturing a 3.6 pack to go in the lower storage compartment would outweigh what they'd get from sales. As for running a charge tank and a power tank located in the lower compartment, again it would just make more sense to slap a charge tank on a 14.4 model and that way you can go even further before you need to charge (or charge for less time).
EDIT: Should've just read Don's post before making mine, it would seem we both think alike