It had an unlikeable lead character
We love Sophia Martin-Green, but it’s fair to say that, at least in season one, they made her very unlikable. For the first time in Star Trek history, the main character was not a captain of a ship or even the commanding officer of the ship, but a war criminal. Early in the show, Burnham would commit mutiny by attacking a Klingon ship.
She would assault her commanding officer, and then fire on the Klingons, which essentially kicked off the conflict with the Klingons into high gear. It was done to show the audience that she understood the enemy more than her commanding officer and that she was tactically a better fit, but it really just ended up making her unlikable.
Star Trek shows have always respected the chain of command, and rarely, if ever, was it challenged. And when it was, it was done out of love. Like when the Voyager bridge crew refused to follow Kathryn Janeway’s orders to leave her behind so she could close a wormhole. They weren’t going to leave her behind, so they refused to obey her orders.
But Burnham’s hubris and over-confidence were not something that fans related with. She defied orders, orders that needed to be followed to save lives; especially when you’re on the brink of war with the Klingons. Burnham’s actions proved her to be selfish and fans caught onto that.
Now, Burnham has been better fleshed out and is a far better likable character now, but as they say, you only get one chance at a first impression and Burnham’s was bad. Tack on the fact that the show really shoehorned a lot of unnecessary concepts into her character (like being Spock’s sister) and fans felt that they had already given her more than enough chances.