That's because Labour have thus far failed to connect with their supporters/failed to get the message out that their official stance is to support remain.
You know why that is? Because Corbyn is a secret Brexit supporter, it's plain to see. Just look at the way he voted in 1975, his quotes about the EU, the way he speaks about it at the moment. Can you imagine any other politician when being asked how important an issue they are campaigning for saying "erm, about 7 and a half out of 10". He clearly sees his primary job (now) as trying to hold the party together.
Quoted from the Spectator:
As one Vote Leave spokesman said, ‘It’s extremely sad to see that Jeremy, who is for all his faults a conviction politician and a lifelong opponent of the EU, has been gagged by the clapped-out Blairites rejected in the Labour leadership contest.’