Because Clefairy's glorious Special Attack won't give your opponent any free room to do work with! Anything will crumble at that magnificent 156 Special Attack firing off unboosted Moonblast! Even Cobalion has a chance to be nearly 4hkod by this beast!
In any case please don't use Clefairy outside of jokes because while it can actually be hard to beat after a few Calm Minds (there is this replay of Molk 6-0ing a known player with this I don't have linked) so is Reuniclus (which has better bulk even with Eviolite and actually hits hard) who also doesn't struggle as much against Mega Steelix's strong Steel-type attacks. Which just blow Clefairy Back.
i was considering not even mentioning Calm Mind, because everyone always has this reaction:
It has a setup move? But its shit at sweeping!!! Git gud bro
setup moves can be used for more than just sweeping, and i'm probably the premier user of this. this is just a PSA about that, open your minds to possibilities. tbh i did mention CM in that way to an extent, however, so let's move on:
my point in mentioning CM was that it's one of a wide array of options you can slap onto the fourth moveslot, which might help out your team from time to time, in this case mostly in degenerated late-game situations or very occasionally against a team weak to it. Not a great option, but my point was that it can do a fair amount of what Aromatisse does, then have a lil' extra on the side. It's a bit enraging that you fixated on Calm Mind; maybe it's not even good enough to be mentioned in my post or used, but what about, well, all the other options mentioned?
Moving on from Calm Mind: if such a weak Moonblast leaves you setup bait, so does being a mon that heals with Wish (and especially WishTect), i.e. aromatisse. Additionally, you can use Thunder Wave to thwart general setup, or Flamethrower / Knock Off to stop more specific setup (flamethrower makes Durantula not want to switch in, and chips doublade, knock off annoys everything but far more relevantly makes Doublade much easier to handle). Encore is even an option, albeit a bit prediction-reliant. Anyhow, Clefable isn't as easy to switch into as Aromatisse, who has only moonblast, possibly paired with CM but still that basic coverage, and very, very rarely Toxic:
Wish 99.167% | Moonblast 97.715% | Protect 92.178% | Aromatherapy 71.963% | Heal Bell 27.032% | Other 11.944%
plus Poisons / Steels / mguard mons who love to switch into Aromatisse still give absolutely no fucks, to top it off if the opp lacks other clerics it's almost always aroma, skews harder to aroma the more defensive the team is.
Meanwhile, at least Clefable might have something up its sleeve. You might go to some neutral pivot, rather than get your wincon smoked. Or, you might get your wincon smoked :DDD
I think more testing needs to be done, and it's definitely lacking in the WishPassing department (i.e. I refuse to consider Wish + Protect a viable option, and will henceforth not acknowledge that it learns Wish at all), and walling Fighters department. But it can be an OK general bulkmon for stall who provides Heal Bell, then either Stealth Rock, or isn't total lolbait like all other stall clerics (togetic, licki, audino, aroma cannot run another another move, Lanturn kinda meh and has no healing itself) since it can run STAB + Recovery + Heal Bell + (Twave / Encore / Flame or knock teamspecific)
for the sake of avoiding future conflicts, i won't mention CM as a potential fourth option ever again. I don't even endorse using it.