So... instead of disproving anything I said, you try and use(bad) sarcasm to call me stupid. That right there is proof enough you don't REALLY know what you're talking about to anybody. You're just full of it.
Sigh. If you insist. Let's look at the "third versions" of the other 4 regions.
Kanto - Blue Version was actually the original third version for the original set (I'm sure you're aware of how the original titles were released in Japan), and lo and behold, Blastoise was used on the cover, as the remaining counterpart to Charizard and Venusaur, who graced the covers of Red Version and Green Version, respectively. And then when it came time to engineer a fourth Kanto game, they picked the next best thing to one of the starter Pokémon, which was, of course, Pikachu.
Hoenn - Kyogre for Sapphire, Groudon for Ruby. Then it came time for Emerald. What ended up in that game's cover? Was it one of the Regis? Nope. It was the obvious choice, Rayquaza. A lord of the sky, to match Kyogre's mastery of the sea and Groudon's rule over the land. Green, to go with blue Kyogre and red Groudon and to match the game's title, "Emerald." Bodily markings very similar to those seen on Kyogre and Groudon. Made perfect sense.
Sinnoh - No third version yet, but it should be obvious that it's Giratina. A partial Dragon-type, just like Dialga and Palkia. Base statistics identical to those of Dialga and Palkia, a name etymology derived from a gemstone, numerous aesthetic similarities; fairly obvious.
Suicune was used as Crystal's box Pokémon simply because there was no third counterpart to Ho-oh and Lugia; no "third bird," as you could say, that was a logical choice. So they did what was necessary and pulled one of the legendary beasts to use and center the story around. This is what's called an aberration, a departure from the norm. It is not the usual circumstance and as such it should never be used as a logical precedent in a debate. You say they should have used Celebi? Absolutely not.
Why, you ask?
Nintendo created this concept of "event Pokémon" to add artificial scarcity, rarity and demand when it comes to a handful of creatures out of the hundreds. They do this to push across the in-universe concept of "there are always some Pokémon so rare, so magnificient, that there are only a few of them." The real-world way of achieving this, obviously, is to ensure that there are some critters that you can only get by venturing out of your house to the nearest toy store and standing in line with hundreds of others like you, hoping for the same thing. And when you inevitably end up spending money on Pokémon merchandise when you're at these events, Nintendo sure doesn't mind.
But guess what; if Nintendo just chucks that out the window and slips an event Pokémon into a game and does not "lock it" with an event item (Eon Ticket, etc), all that scarcity disappears and Nintendo loses the artificial demand they create for certain creatures, among other things. Notice how whatever circumstances allowed people to get Celebi from a Japanese copy of Crystal were removed outright for the American release? That wasn't a random decision in the slightest, although it had quite a bit to do with the removal of the game's support for the dial-up online adapter that the Japanese version had.
This is why putting Celebi on the cover of Crystal would have been illogical to the supreme, and why putting Arceus, or Shaymin, or Darkrai or whoever on the cover of the third Sinnoh game is just as illogical. Nintendo tends to this franchise
very carefully; again, they are not going to create artificial demand and mystique for some element of it and then just chuck all that out the window on a whim.
