I don't think it's anything to do with any actual law, rather a push by the liquid manufacturers a while back to the lower mg due to wanting acceptance and less likelihood of being accused of selling a dangerous (ie too strong) product.
The liquid comes from a supplier that has implemented this policy, hence the drop in availability of 36mg/ml liquids you've seen. The cartos are subject to this same lack of availability though the cartomizer manufacturers themselves are willing to put whatever label on it we want.
There are higher mg liquids available but for the most part it is unflavored and the producers of it don't have the wide variety of flavors you're used to seeing. If 36mg was available we'd have them as the 36mg liquid (which we still have some stock of) was a relatively popular seller and we understand that there is a good market for it.
I'll check deeper into it and with some more vendors but for now I'm just reporting what I've encountered thus far.. The simplest explanation is usually the right one