Probably a lot of reasons, but mostly demand, I think. I like a lot of games on ST-V. Some of them I like very much. But I don't think any of them quite make my top 50 favorites of all time, whereas a bunch of Neo games do.
The hardware is also cool. It's not hard to switch out an ST-V cart, but there's something cool about only having to hit a select button to be able to choose any of 4 or 6 games without even turning off the cab. I'm not sure what the appeal of a 1-slot MVS board is.
The closest equivalent to MVS games would be CPS-2, and from what I've seen, in general they tend to actually be more expensive than MVS carts, I think (even without the A board).