What market share does Jet A-1 have in the commercial airline market?
Chevron writes

Jet A is used in the United States while most of the rest of the world uses Jet A-1. The important difference between the two fuels is that Jet A-1 has a lower maximum freezing point than Jet A (Jet A: –40°C, Jet A-1: –47°C). The lower freezing point makes Jet A-1 more suitable for long international flights, especially on polar routes during the winter.
However, the lower freezing point comes at a price. Other variables being constant, a refinery can produce a few percent more Jet A than Jet A-1 because the higher freezing point allows the incorporation of more higher boiling
components, which in turn, permits the use of a broader distillation cut. The choice of Jet A for use in the United States is driven by concerns about fuel price and availability. Many years of experience have shown that Jet A is suitable for use in the United States.

Notably, the US is also the single largest market for aviation fuel worldwide, so it has the volume to support a separate type. There's consumption numbers in the same document.
The disadvantage with Jet A is with long-haul flights that experience cold air, the 7 degrees can make a difference, forcing the flight to descend to warmer levels to heat the fuel, increasing consumption.
One thing that can be done is a sample of the loaded fuel is taken and the actual freeze point determined, and that number used in flight. Usually for Jet-A this lies somewhere in between the two specs.