You have to remember, Riot is a business. Pretty much every decision they make has to go through the lense of “will this benefit the business?”
LoL already has the largest player base, not just of any MOBA but of any online game period. They have probably reached the point where they are no longer going to gain new players just because of gameplay improvements. At least not without also alienating a part of their current player base. But a business needs to grow. That’s basically the fundamental tenet of all businesses. If you aren’t growing you’re dying.
So instead they have to look to other outlets to keep people playing so they can potentially spend money on the product, either directly or indirectly.
And personality is quite frankly a proven method of drawing player support. Think of how the Overwatch fan base goes crazy over every released video and comic. How there are people who bought the game because they wanted to play the talking gorilla or the cyborg ninja. People respond to stories and designs basically by our very nature.
And what’s more, expanding the narrative design of a game is probably one of the cheaper methods of increasing player retention. I don’t have any hard numbers on me, but hiring a few writers probably took less money from Riot than something that requires coding and rebalancing like the new rune system.