This is just an educated guess.
In red and blue, we had sound technology limited to beeps and more beeps. As technology improves, Pokemon cries are of better quality. As a result, the beeps and boops of the 1990s sound out of place next to the high tech cries of tommorowland. So something had to be done sooner or later. And now, this generation of Pokemon gave these Pokemon high tech cries as so to better improve your playing experience. (Those beeps and boops are not much pleasant to hear anymore)
But hey, that's just a theory. A GAME THEORY. Thanks for looking (at this).