Reminding the electorate of a pledge that you freely made and then abandoned isn't great politics. The apology isn't about the policy, the apology is for making the pledge in the first place. All you have done is remind people of exactly why they deserted your party in the first place. They aren't going to come back.
This has reopened a whole can of worms about why your party made a pledge to support a policy that you knew at the time to be unaffordable. It was a publicity stunt, pure and simple.
"I shouldn't have committed to a policy that was so expensive when there was no money around"Danny Alexander told Clegg in March 2010 that the policy was unaffordable and Vince Cable is known to have opposed the stunt, but every LibDem MP signed up to a very clear promise that most of them were promptly to break.
As an aside, it is interesting that Clegg uses 'we' throughout much of the piece, but apologises in the first person for committing to the policy.
Here's Nick in happier times, promising to keep his promises.