Re: In-Universe Explanation for TOS Retro Tech
Non-canonically, I believe the Romulan War novels explain that the onboard technology was regressed to harden it against a telecapture system that the Romulans had been using against Earth vessels.
I don't think an explanation is really necessary and would be completely unsatisfying anyway. Sometimes it's more fun for the man behind the curtain to remain behind the curtain.
"If I hadn't tried, the cost would have been my soul." - Admiral James T. Kirk, Star Trek III: The Search for Spock