[Tobes]

It is much more likely that the opponent is going to lead with their non-Magic Bouncer so that they can increase the possibility of you deciding to go for hazards, giving them a better chance at a reward for the switch-in. This is especially true if they use something like Genesect. You don't like the possibility of Shadow Balling to hit the switch only to eat a U-turn, and don't like the prospect of double-switching since U-turn also makes that risky. Jelli, a team with a Magic Bouncer is probably more comfortable leading with something that beats prospective leads other than Deoxys-S, since Xatu can generally switch into it for free anyway.

Furthermore, unlike in OU, Xatu is really the only proven Magic Bouncer, since Espeon's viability is questionable, and Xatu is almost always physically defensive to better combat Groudon and Ferrothorn. Considering all of this, as well as the threat Deoxys-A and Giratina-O pose, Shadow Ball Dark Pulse is definitely the more practical move, by a wide margin.