Probably because nic salts start at around 25 to 30 mg. Those are popular as well.
Another thing I have noticed (I have two vendors for shipping, and one local store) - there were plenty of 12 mg vape juices prior to Monday,. I can't find them anymore. In fact, a few lower nic juices I did order are page not found. Depending on where the vapes are made, and supply issues, plus the new restrictions, some are not making the goods anymore, and just letting them die off?
As to 18mg, I haven't seen it anywhere since returning to vape. It is not a perfect world anymore.