Checkboard - regardless of what people think, I love the basic simplicity of the US system coupled with the philosophy behind it.
If I can try and put that in a few simple phrases:
1. They believe in the see and avoid principle of aircraft separation in VMC, they believe IFR flights and associated procedures should always be conducted in controlled airspace'
2. They, therefore, start with a big "bath" of Class E airspace, controlled for IFR but not for VFR, but with weather requirements for VFR pilots that enable all pilots to have a good chance of seeing each other in VMC
3. If the traffic gets too dense around a particular aerodrome they add Class D airspace - a controller then assists pilots to separate by sequencing in the circuit and along the runway(s)
4. If the traffic needs sequencing beyond ATC Tower visual range they add Class C airspace - but note that they then also require surveillance. This enables the vectoring of IFR flights while VFR flights are still able to self-separate. This works well where there is a main instrument runway (Vectored IFR aircraft) and secondary runways for VFR flights (handled by the Tower ATCs)
5. Class B airspace may then be needed where VFR aircraft need to be vectored for sequencing with IFR aircraft due to runway saturation - VFR flyways or corridors are generally incorporated to allow the free flow of VFR aircraft not using the aerodrome
6. I doubt that the US would ever contemplate Class A airspace where VFR flight is possible, and even Europe has now given that idea away.
So the airspace type is not dictated by RPT movements or passenger enplanements, as it is in Australia, the airspace is determined by the type of ATC needed to efficiently and safely manage the airspace; as the risk of collision increases so does the degree of ATC assistance.
Simple really but first you need some design principles - our airspace is designed by a committee, the same one that created a camel out of a racehorse!