Because he's the only MP who could enter this People's vote and have a chance of somehow "losing" it. His place of mind comes from a euro-sceptic and it just remains challenging to believe he's truly backing some of what he says.
The smaller opposition parties have handled themselves better than Labour around this.
Calling out Corbyn around Brexit doesn't mean throwing him under a bus, full stop.
May voted remain, she's leading the leave campaign, Corbyn probably voted leave, he's leading the "remain", or at least, soft-Brexit campaign.