2 generic types were by designed not supported because with 2 generic types the amount of AOT classes that would need to be generated for AOT platforms would be literally thousands (due to all the different type combinations possible). Thus only 1 generic type is supported so that it is also possible to properly support AOT platforms where generics is a problem and AOT classes have to be generated. So since that was a limitation early on, many other things in FC were also designed based on the fact that there will be a 1 generic limitation.
Gavalakis
