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.
"I had no idea you were so... formidable. " - Anan 7 to James T. Kirk, A Taste of Armageddon