I'd put it down to his press team as they're probably the ones deciding this.
The only view I can think of which is at all charitable on this decision is they realise there's nothing they can do to stop the looming electoral crash and they're now just trying to minimise the effect of it. So they decided that they don't want to give any wavering voters a reminder that there's other parties they can vote for apart from Tory and Labour. Rather pathetic really if that's the case.
A bit harsh blaming it on his media team. He is ultimately responsible for strategy. He's presiding over a train wreck. If he tries to hold on after a disastrous defeat it says a lot about him. Mainly that he's a selfish idiot.