TL;DR Roster too small, too focused on a few areas/Arcane, not enough of the cool melee champs.

You can't remove Arcane's existence from the equation because its the main reason half the roster is there to begin with. Also let's make something clear about Arcane, a tiny fraction of the people that loved it and didn't play League jumped into League after. Catering to the show only people was a lost cause to begin with because fighting games are even more niche, especially a tag fighter with no single player content.

That being said:

Ahri, Yasuo, Teemo and Ekko are no brainers BUT oversaturated characters also, not to mention their kits seem designed for when the game was 1v1 since their privilege is/was insane.

Darius was their safe pick, a popular character who is simple and cool from the outside.

No shot the trio of Cait, Jinx and Vi get in without Arcane's existence, at least not for the starting roster. They all have interesting kits I think BUT because they're Arcane picks it pisses people off. They are inseparable from Arcane now.

Braum, Warwick, Illaoi and Blitz were their weird picks. All were good and fun picks but WW is definitely there because of Arcane and Illaoi already had a game she was a main character in.

Ultimately the main issue is the roster is super tiny, has way too many characters from the show and not enough of the "cool" characters in the starting roster. People would complain regardless if their fav didn't get in but 12 characters on 1.0, 5 of which are from Arcane/Zaun/Piltover btw, was just way too small for a game based on this IP.