Guaranteeing an in/out vote would have shifted the debate back onto Tory territory and could have wrecked a future Miliband premiership.
Beyond the bluster and rhetoric, there is a surprising degree of consensus on the reforms needed.
It will become harder for the PM to insist he can succeed when the europhile and the europhobe both declare he will fail.
Miliband doesn't want to make a pledge that raises more questions than it answers.
The Prime Minister can see the strategic as well as the economic logic that keeps Britain in Europe.
After withdrawing from the centre-right European People's Party grouping, Cameron has no right to tell his MEPs not to flirt with the anti-Euro Alternative für Deutschland.
A public vote offers the best prospect of responding to democratic alienation from the union, and establishing a secure platform for the UK's engagement in its future.
His party wants a eurosceptic but the PM may decide that he needs a business figure with a record of constructive engagement with Brussels.
UKIP trails Labour by six points but, as in 2009, the party is hoping for a late surge in the polls.
Promising an in/out vote would shift the debate back onto Tory territory and could wreck a future Miliband premiership.