Many? Milotic was once, Metagross was twice, Dragonite was once.
And...
Milotic was only distributed in Kyushu, and was part of a random distribution.
Metagross was a part of that same distribution as well as another random distribution at Pokemon Centers.
And Dragonite was a part of that same Pokemon Center distribution.
So it shouldn't be said that everyone already has these Pokemon. Even assuming everyone was able to attend, there's no promise that they'd get any of those, specifically.
But then... the same can be said for Garchomp.
Spritomb seems to be more popular in merchandising than Garchomp these days, anyway.
Or it could be because Spiritomb is still pretty hard to obtain, compared with Gabite. There's no place to catch wild ones, and it's only been available through that tedious quest in DPP, the Pokewalker, Dream World, Dream Radar, or an in-game trade in B2W2. There's no easy way to just catch them freely, most methods require outside devices/methods. So maybe that had some reasoning? To make it more appealing than Garchomp? Could also be because there were already two dragon-type Pokemon in this distribution from the specifically dragon-type Champions. Just some ideas.