First of all, all their CGs in RPGmaker releases were downscaled (with full version released separately as bonus to the game...) and even despite that the games still run poorly. I'm pretty sure the dev also said he had issues implementing high resolution animations to mid combat scenes (and mixing them with static CGs). This is not only OneOne1's issue as Vitamin Quest 2nd Attract runs like shit, too.
Also, isn't VX Ace only 32bit? This alone means it's basically dead. MV is old, too. Considering OneOne1 would have to learn from scratch (Ace uses Ruby while MV/MZ is javascript) it's not worth it.
They are using RPGmaker assets because they already have license for them. So why not?
MV is so awesome everybody hated it when it came out and devs still hate this to this day because of limitations and inflexibility. Go and take a look at any more "advanced" MV game, for example Magical Girl Celesphonia and how heavy it's with scripting. It's basically spaghetti code. Not to mention - different language version are not compatible in any way, including saves.
A lot can be achieved in MV - but at what development cost? Each time you want to stray from the jRPG path and add some new idea/system to your game, it's a huge hassle.
RPGmaker brought us many great releases but good games do not equal good engine. Same goes for Unity - many trash releases don't mean the engine is bad. It's quite the opposite - there are many bad releases, because it's easy to use so devs just go for it.
I mean, how many award-winning indie games use Unity? Hollow Knight, Ori series, Cuphead, fucking Genshin Impact...