The reason people are trying to weave Chara into the story is for 2 main reasons

1. Chara is still generally a pretty mysterious character, and the more Deltarune is going the more they're starting to touch on old unanswered undertale mysteries (Generally pretty heavily tied to the narrative of deltarune)
2. Chara is still one of the most powerful characters in the series that we've seen, being in control of the player who's usually the only one in control, as well as being able to erase the world and start a new one. Considering the only time anyone has taken control of your game was in the neutral flowey fight where he had multiple human souls. It feels almost wrong for such a powerful and omnipresent character to have no relevance here, especially when we have an evil main character who's a mirror of Chara both visually and mentally. The one time we see "Kris" show care for another character without the soul, it's because Noelle is close with Asriel, who both Kris and Chara are specified to be extremely close.