From the very beginning, the show sets up the main conflict with the Upside Down forces (Mind Flayer, Demogorgons, etc.) against El. She's the one who opens the gate. She's the one in season two who has to close it. She's the one in season three who the Mind Flayer is coming after. Flayed!Billy specifically says, "We built this for you." Then in season four when they add in Vecna, they give all the backstory and connection to him to El, and the big final fight is mainly between the two of them. Because she's the main character and always has been.
But this season, since they decided to sideline El, the focus is now on Will and his connection to Vecna, just like the focus is on him and his powers.
I still can't believe the writers thought, "Hmm, I know we've made this girl the main character for FOUR WHOLE SEASONS, but let's shove her aside and barely spend any time giving her anything, barely give her any interactions with her boyfriend, only let her really spend time around her dad repeating conversations and give her a TINY bit with two other characters, one of whom is her sister, but instead of them reconnecting and forging a relationship, said sister wants them to kill themselves, and let's ignore all of the stuff we did last season with expanding her powers and all of her training over the last eighteen months and make it so she struggles to lift a truck, something she was doing back in season one, and let's ALSO ignore how Max is one of the people she's closest to, someone who played a big role in her story, someone she BROUGHT BACK TO LIFE, in favour of actually making Max besties with Will. Because he's suddenly the main character now who Vecna is seeking out, who gets to go against Vecna and explore his powers, despite him being almost a secondary character for most of the show. That's a great idea! Let's shove the main female character aside for this guy who spent half the show as a plot device and the other half lurking in the background."
We can talk all day about the other issues with the season (pacing, stuff needing to be cut down, focusing too much on Holly, neglecting other main characters and dynamics, etc.), but the biggest problem starts with the decision to bump El down and give Will her place.