As a start, please see my papers referenced below. It is my position that the deregulation of the electricity markets has increased financial risk as perceived by lenders and equity investors. This is the primary reason behind your observation. For example, there are 441 nuclear plants around the world, all of which were financed under one or another form of the "sovereign" method of financing. There have been zero nuclear units financed within deregulated competitive markets because the underlying credit support has been removed. Thus, I do not believe it is due to your third choice (loss of knowledge) as there is significant ongoing research in nuclear engineering within the US as well as some intriguing new modular reactor designs. However, these new designs do not alter the impact of deregulation on financial risk. Your first choice (economics) has been a contributing factor, but it is your second choice (politics and economics) that led to deregulation, and deregulation led to increased perceived financial risk.
Dansky, S. (1994). Deregulation and the end of project financing. Public Utilities Fortnightly,132(14), 17.
Dansky, S. (2002). Is deregulation the problem? Public Utilities Fortnightly, 140(18), 10.
Dansky, S. & Cudmore, B. A. (2022). Marketing challenges for the next generation of nuclear power: Has deregulation eliminated the nuclear option? Proceedings of the 2022 International IEMS Conference, Ed. M. Edwin Sawan, Cocoa Beach, Florida, March 14-15, 2022, p.88-100. ISSN:2690-3210 (print), 2690-3229 (online). https://soar.wichita.edu/handle/10057/24229