Because he was lumping in aimless ball after aimless ball into the box! Walker is usually the last man when playing for City, his role is to cover the ball over the top of their highline, he's not relied largely on attacking phases (Pep does mix this up a bit but when Walker is asked to join attacks he dose this through the middle, not from out wide and therefore HE DOESN'T find himself out wide in crossing positions, he's in midfield). Emerson is currently pissing all over Cancello's stats!!!
He didn't fit our style, Poch is a possession manager, he builds attacks from playing out from the back. Tripps can't do this, it's one of the biggest reasons that made us shit (not a sole reason).
When he crossed we lost possession of the ball and were counter-attacked repeatedly with players out of position up the pitch.
He disrupted the balance of the team, simply unable to do all the things that Poch wanted, it's why he was literally the first out of the door when Poch got the chance.
I'm glad you've now come around to the notion that he can't defend, something that you repeated said in the past that he could. He was so shit he was constantly targeted and probed as our weak link, didn't have the recovery to get back when a ball played behind him, and if he was in defensive positions he was usually dribbled past whilst holding his face, rolling around on the grass whilst the oppo was scoring goals for fun.
The only team he's looked comfortable playing in has been England. Slow, passive England before they upgraded to more athletic powerful and dominant FB's that we have now.