S 7.2 $10,995
SR 14.4 $15,495
Extra battery would add $2,500:
S 14.4 $13,495
That does seem to fill the gap nicely, so it's surprising they don't offer it. Even if they don't advertise it, they should still allow people to custom order it.
It obviously had to do with how Zero wants to market their bikes in the U.S. My guess is that they want the SR/F and S models to appeal to the longer range customer while focusing the smaller battery and less expensive models to commuters. Zero may feel that not having a 14.4 S in the U.S. market likely increases the numbers of the newer SR/F and SR/S platforms that they sell here.
I think you're onto something here. They're trying to allocate the market across a set of configurations that will maximize profits. A 14.4 S could possibly cannibalize sales of more expensive models. It could also appear under-powered for the weight, get reviewed by the wrong person, and compare poorly with ICE bikes in its class.