It has nothing to do with the nm in the chips, it's all about reward and availability
The fabrication process invokes moore's law.
A smaller nm semiconductor process gives you a higher hash rate and lower electricity consumption. Have you ever wondered how new generations of ASICs produce greater hash rates @ decreased energy consumption? Its due to the downsizing in nm scaling.
This observation holds true with both ASICs and GPUs. It plays a big factor in how competitive GPUs are with ASICs in terms of mining profitability.
Availability and backlog of orders might be a valid point. That hasn't been an issue in a long time though. Its more of a historical myth that hasn't necessarily reflected the condition of markets in awhile.
I've read it somewhere on the Web about a year ago (maybe a little over a year) that ASIC's had already reached their maximum theoretical capacity (in terms of hashrate) due to technological limits imposed by the semiconductor process based on silicon. It has been the case with CPU's for almost a decade already. Processing frequencies no longer grow as you can't indefinitely diminish the size of the tranny. Yeah, I know there are a host of other factors at play here, but sooner or later stray quantum effects invariably kick in. And with CPU's this threshold seems to have already been reached. Perhaps, it is the same with GPU's as well.