I'm pretty sure OneOne1 explained "why Unity" when they were developing Samurai Vandalism. TL;DR RPGmaker is a pain in a dick and it was not worth to learn javascript from scratch just to use game engine that's already pretty old and has some serious limitations.
From my personal experience: there's not a single 2+ GB RPGM game that runs smoothly and/or has assets in higher resolution than 720/768/900p. Then, I never saw a RPGM that uses cut-ins, animations and static CGs at the same time, all for battle fuck events.
Samurai Vandalism was crap when it comes to technical aspects but I didn't have a single problem or crash in Eclipse Witch and it plays smoothly even on my potato PC (5 yo pentium and 1050ti).