I can see the logic behind not giving Pokemon with Levitate an alternate ability because, well... Pokemon like Weezing levitate. It's a pretty integral part of their design. It felt kind of dumb to me when they gave Weezing a second and third ability, even though both of Galarian Weezing's other options are great.
I see nothing wrong with Levitate users getting other abilities because since the introduction of Abilities many Pokemon that would seem like they would be immune to Ground(Like the Magnemite line and all of the winged Bug/Poison types) can't get the ability at all.