Riot said a long time ago back in like Season 5 that Riot has designated Flat Mana as a "Mage" stat and Mana Regen as a "Support" stat.
The reason is that Riot doesnt like Mages ignoring their mana bars and effectively buying their way into being resourcesless, as it effectively ruins the point of mana management, and on a class that deal huge AoE damage and have a lot of wave clear and chunking, this can be problematic for seiges, stalling out games, etc.
Supports on the other hand give up a lot of things such as damage and wave clear in order to stack mana regen now, so they see it as a fair trade off for it since they are dedicating themselves more to utility.
This change goes hand in hand with this.
It also encourages more mana stacking, giving you a large reserve instead of simply mever running out, and encouraging otems that scale with and give more mana.
By lowering mana regen and raising base mana, they also give them more "ammo" to compensate and also promote better mana management.