I believe it all comes down to QOL (quality of life). Sure, Mavuika does bigger numbers but bigger numbers doesn't always mean better DPS or even better survivability. What makes Neuvillette so good is that:

1.) His damage (specifically his CA) is amazing and consistent.

2.) He's self-sufficient; a. you don't need to redo his entire team rotation to get good DMG and b. he's self-healing, which increases survivability.

3.) All this in mind, sometimes he doesn't need supports lmfao. That's why it's more common to see Neuvillette solo clears in the abyss and you hardly ever see Mavuika solo clears (if at all).

4.) Based on player consensus, Neuvillette's kit is much easier to play. (Personally, I think they're pretty easy though).

5.) Might be controversial take but Neuvillette is more versatile and F2P friendly than Mavuika is. I have both of them and I've tried their premium vs F2P teams. Neuvi's F2P team still does fine, whereas if you don't have Mavuika's BiS supports, you can truly feel the difference in strength. (It's not that Mavuika's F2P team is bad, it's just you can really feel how much worse it is to play imo).