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.