It's because the public don't trust the party with Brexit...
... Mostly because Labour MPs are in a mixture of seats that voted remain and leave. They, more than any other party, are split by Brexit, with many Labour MPs tentative about being a pro-Brexit, screw-the-eu-it-was-no-good-for-us-anyway gang.
But the public do trust Labour with social care and the NHS.
Of course trying to find a way to make the election about social care, without then having the argument pivot to "Labour always wreck the economy with irresponsible spending on social care" is another challenge.
Corbyn may have come up with a decent reply in today's Prime Minister's Questions:
Only this year the new Chancellor pledged to eradicate the deficit by 2022.
I admire Tory consistency: it is always five years in the future. Another Tory broken promise.
The Prime Minister leads a Government who have increased national debt by £700 billion, more than every Labour Government in history put together.
Debt has risen every year they have been in office.