"A lot of those projects have now come to fruition or are on site, and it seemed like a good time to move on to new challenges. It is also a good time for someone else to come in and take them to the next level.
"The Big Dig is part of life when you are regenerating a great city, and with 2008 as a deadline it gave us a much shorter timeframe than is normally the case."
He also played down any concerns about his new job constituting a conflict of interest given the Enterprise-Liverpool contracts with the city.
"My new job at Enterprise will be based in Manchester and with a national brief, and as such it will not involve me in anything to do with Liverpool, as that would be highly inappropriate," said Mr Parker.
"It is a difficult time when the governance of the city is caught up in the sorts of issues we have seen recently, but the timing of my departure is coincidental."
But, as Mr Parker headed for the riot-torn French capital, his decision to hand in his notice left a trail of questions from opposition councillors in Liverpool.
He was being viewed increasingly as a key contender for the top job as chief executive of the council in the event of Sir David Henshaw leaving.
As regeneration cabinet member Cllr Peter Millea heaped praise on the work of Mr Parker, opposition leader Cllr Joe Anderson claimed: "This is the crumbling of Sir David Henshaw's dream team.
"Mr Parker's comments confirm what we've known for a long time, that all is not well in this council."
Mr Parker arrived at the city council in the late 1990s after spending four years with the highly successful Speke Garston Development Company.