Because you can end up in a situation where legislatures want to do things that are not supported by a majority. A proposition that changes statute is ripe for reversal, while a constitutional amendment is not.
What the Ohio GOP was angling for was “hey, we know a majority doesn’t support us, but we have that handled by gerrymandering, so now we need to make sure we enshrine minority rule by allowing 40% of voters to control the democratic process.” They know they can keep 40% gaslit and brainwashed, but getting to 50% is a challenge that requires policy instead of bombast.
And it was done precisely because a popular (by polling data) amendment is coming this fall that takes away the GOP’s power to control people’s bodies.
Thus, this election brings into specific relief why 50% + 1 is the only way to protect voters from legislative overreach. Now, and going forward.