>>45135864A reminder that Leaf is essentially a Gen 3 character.
As for your question, they should have introduced her in the anime in the Battle Frontier season when she FireRed was relevant, in 2020 she just doesn't matter that much since we're deep into Gen 8 with so much protagonists you'd need a notebook to memorize all of them. No proper media exposure (compared to the likes of say, Serena) = no long lasting popularity and fanbase.