Why TOS And TNG Romulans Look Different

Both Romulans and Klingons underwent an appearance change, the former occurred between the TOS movies and TNG, whereas the latter occurred between TOS and the first of the TOS movies. A canon explanation was created for the Klingons in STE, but there is nothing for the Romulans. Of course I have no canon authority, but what I can do is derive an in-universe explanation by treating the fictitious universe as actual history.