>>44116069That's because sinnoh is like the climatic end to pokemon lore, throughout the games we started with the current, the birds and mewtwo, genetics and whatnot and slowly went back in time, first with lugia, ho-oh, creatures that long abandoned us, then the super ancient pokemon, groudon and kyogre, the regis, jirachi(I think idr much but it sleeps a long time). It was topped off with origin itself, gods of space, time, emotion, the creator of all things, and the devil.
Now, where do you go after you covered the very beginning?
logically you should go forward with the present to the future. That's why a lot of the memorable later legends are futuristic or current in nature. No one wants some other old ancient pokemon with some made up significance, we already have those roles filled.