Pokemon, what happened to you?

[TomGuycott]

Really, a lot of recent Legendaries are just marketing ploys or tie-ins to other Pokemon spin-offs, like Ranger and Mystery Dungeon. Lots of Legendaries become the focus of Pokemon movies. As for the tie-ins, take Manaphy, who actually DEBUTED in a spin-off game before Gen 4 was released. Then take Shaymin, who ended up being a big focus in a spin-off, granted after it's release.

In other words, the Legendaries are marketing gimmicks, and they try to draw people into events with them in order to get them. So they're just gonna make more and more until each game comes with 100 new Legendaries.