Larian revealed once that 90% of players make a custom character, which is Tav. Why neglect the character that is played most often and spend most work on Karlach Origin that is played by less than 10% of players?

I really dislike mary sue characters, which is why I'm not a big Karlach fan. I mean if a writer enjoys writing a mary sue that's fine but not when it comes at the expense of other characters.

While I like the epilogue with romanced Astarion before the epilogue party, the dialogue seems not as sincere or loving as in Karlach Origin and it is really disappointing. Why does a niche origin character get so much more work compared to what most people are playing?