Two reasons:
Soft-Reboot, which essentially means they're going back to basics and trying out new styles both in animation/art design and storytelling, but they don't want to start from the very beginning. The SM anime artstyle more closely resembles the artstyle used in the first run of the Pokemon manga, which is likely where the inspiration came from.
Easier to animate. Simpler character designers are easier to animate in complicated ways. Even if you think that the older designs looked nicer, they also resulted in the character's movements looking basic and stiff, while simpler designs allow for more complex and expressive movement, resulting in the show looking better in movement.
It's nothing to do with Pokemon but this clip here illustrates exactly what I mean when I talk about complex character designs stifling movement, and making easier character designs makes movement more fluid and expressive.
