Because he has allowed Theresa May to do whatever the fuck she wants, even though the majority of voters don't want hard Brexit and the overwhelming majority of Labour voters don't want to leave, even though he knows hard Brexit will fuck over the people he claims to care the most about, all because he is too scared to challenge his own ideology. And on an unrelated note, his near-silence on the Trident malfunction fiasco despite being a lifelong anti-nuclear campaigner just adds to his feeble record.
I'm sorry but he absolutely needs to go.