The guy above said that its to drag the development out, which might be true but you can play the game with the mouse now. Which is a win lol. but having to play through the game again when it isn't finished again bothered me a bit... xd
I know nothing of this subject but google told me:
The main difference is probably distribution/building: you can only release games for PC with VX Ace, while MV allows you to release also for mobile and for browser.
Graphic assets are also different: VX Ace uses 32x32 tiles, while MV 48x48; and MV resolution is higher, so VX Ace games will look more "retro" and "pixelated" than those in MV.
MV also has a better management of plugins and add-ons, while VX Ace still relies on scripts.
I used both extensively, and I must say I prefer MV, but VX Ace is perfectly fine too. My preference goes to MV just because it's more recent, but Ace is still a valid option, and it might be the better alternative if you can get it for cheap.
My advice is: find a couple of games created either with Ace and MV and with mostly default assets, then try and compare them, so that you can choose the engine you liked the most.
Yes i copy pasted this off a random reddit post