This is from a Reddit Ask Science thread - in the /r/AskScience sub-Reddit the only answers permitted are accurate, and in-depth explanations.
There are responses from ex-Intel process engineers, electrical engineers, nantechnologists, etc. (they talk of CMOS, Moore, specifics of size reductions, etc).
https://www.reddit.com/r/askscience/comments/6t7bdh/why_does_it_take_multiple_years_to_develop/