with low light performance of the 5n, what is the appeal of full-frame?

One question I keep having, but not finding a good answer to, is why it seems so much easier to make high IQ lenses at good prices for FF than for NEX and m43? Why for example is the 35mm f1.4 Zeiss >$1k when I can have a lens with similar IQ and DOF control (f1.8 in FF?) for a few hundred buck?

And why can Nikon make a high IQ $500 24-85mm f3.5-4.5 zoom but there exists no $500 16-50mm f2.2-3 zoom for NEX?

Is it easier to make high IQ fast lenses for SLR's because of the larger distance from the rear lens element to the sensor??