While the frustration of high-nic vapers is understandable, so is the position of vape shops. You can't blame a company for not carrying a product that doesn't sell well. Keeping stock around on the hopes of catching a few people that might want it isn't a good idea for a company wanting to stay in business.
Vape shops in my area sell out of 6mg extremely quickly. Then the 12mg is next. Very rarely do I actually buy a bottle of 6mg juice, I usually have to buy multiple bottles of the same flavor and mix them to achieve my preferred 6mg. There is always some 18mg and usually some zero sitting on the shelf, but 6mg is more rare because it sells so fast. Judging by my juice orders online it's the same almost everywhere, as even web retailers are often out of 6mg but have other strengths.
Basically it's an issue of a small minority not getting what they want (high nic juice) so that the vast majority do get what they want (low nic juice) It's already hard to keep enough popular low-nic juice on the shelf, if they cut down that shelf space even more to provide higher nic juices for a few, then even greater numbers of people wont be able to get their desired nic level, which I thought we wanted to avoid!
If the nic content of your juice isn't high enough, all you have to do is add some 100mg nic base which will be a very tiny percentage of overall volume If your nic content is too high, you're pretty much screwed. You can mix different strengths, but then you have to hope the shop has the appropriate combination to reach the level you want. Not to mention spending 60-100 bucks on one flavor of juice because you had to buy multiple bottles. You can cut it with straight PG or VG, but then you sacrificing significant amounts of flavor.
I know it sucks, but it's not like you don't have options. Either way someone isn't getting what they want, at least the way it is now the majority is still happy.