Which comes with drawbacks. Video is sent out compressed over USB protocol which requires CPU cycles, more resolution + moving images = more CPU grunt required. Plus there is the risk some update will break the driver making your dock useless.
This limitation is purely artificial/for market segmentation purposes.
I don't know anyone who went "Ah! only one monitor support!? I better move up the range to get that 2nd display support..." etc. You would really need to get into the weeds on the spec sheets to notice ahead of purchase.
I actually find many more people doing the opposite - choosing to move "down range" to the Air for the form factor, given it still has a great CPU. These power users who choose based on the great form factor get hurt the worst on this.
It's still for market segmentation but there is a hardware reason.