I've always figured the increases of Legendaries was because Pokemon The First Movie, Mewtwo Strikes Back made millions of dollars its opening week. That's why every movie is about a legendary. They must of figured Legendaries were the key to success, so they kept making them, and pushing them to churn out movies for yearly.
Sort of. It doesn't apply to ally legendaries, they didn't make things like Heatran or Regigigas specifically be in a movie, but it applies to some legendaries including almost all the event-exclusive ones. Lugia was actually made for the 2nd movie because the anime studio requested something from Game Freak to work with. GF originally just planned for Ho-oh fill the Mewtwo role for GS, but later decided to use Lugia as a counterpart to it and make each one a mascot for one version. They didn't have anything else planned out for Gen 2, which is why the anime studio arbitrarily went with Entei for the 3rd movie, and Celebi was supposed to a whole arc in the show (it was supposed to be in the GS Ball), but they scrapped the idea so they could use it as the star for the 4th movie. And then from what I can tell from circumstantial evidence, the Latis were probably designed for the 5th movie which is why they're kind of tacked on to RS and they have the light reference to the movie plot with the Soul Dew item
Starting with Gen 3 is when GF started making event Pokemon specifically to be in movies and be given out in events around those movies' release. They made two event legendaries, Jirachi and Deoxys, because they originally expected the Gen 4 game would be out in 2005, meaning Gen 3 would have three movies - Jirachi movie, Deoxys movie, and Gen 4 preview Pokemon movie. But they had trouble getting used to the DS and DP got delayed a year, so they're ended up being two Gen 3 movies starting Gen 4 Pokemon. First Lucario, who I think was designed as just a game design first but it was an early one they though would work as a movie star. And then Manaphy was revealed in an interview to be a random unfinished design that Sugimori had lying around that the picked to make into an event legendary because the anime studio asked for something for the 9th movie.
Then there were three other event legendaries in Gen 4 - Darkrai, Shaymin, and Arceus - to make sure they'd have enough this time. Gen 4 lasted four years, so it worked out with each getting a move and then there was the Zoroark movie to preview Gen 5. Not sure if Zoroark was made with a movie in mind, but considering it's kind of a Lucario clone it might have been. Then they made four event legendaries in Gen 5 to make sure they had enough, but Gen 5 only lasted three years so they actually went overkill there. Victini, Keldeo, and Genesect got movies, but Meloetta was just given the focus in the Pikachu short prior to Keldeo's movie and in a few episodes of the show, and was given out the same year as Keldeo. And Genesect's movie doubled as the Gen 6 preview movie.
So I guess they decided not to go overboard for Gen 6 and just made three - Diancie, Hoopa, and Volcanion - which was enough for the three years Gen 6 lasted, with Volcanion's movie doubling as the Gen 7 preview by having Magearna. And Magearna was really obviously designed for the movie since it's loosely related to Kalos lore and doesn't fit in Alola at all. And then I'm guessing for SM they knew ahead of time they would probably start introducing new Pokemon mid-gen so they made just the one event legendary, Marshadow. And then added Zeraora in USUM to be the star in the 21st movie. I'm guessing GF wasn't really aware of how the anime makers were planning to do something different the 20th movie though, because GF designed Marshadow to fit with Alola while the movie is a Kanto nostalgia thing where it doesn't fit at all. You can really tell the anime writers didn't consider Marshadow and Zeraora at all when coming up with the plots to those movies and were willing to just throw whatever GF gave them in there regardless (I think the same could be said for some easiler movies too).
GF hasn't really made any more event exclusive legendaries after Zarude, and you can tell it's because the anime movies stopped getting made so there was no longer any real point to them.
