Not sure if it was a lack of courage that prevented a challenge. Howard had a lot of support within Liberal ranks. If Costello could not get the numbers to warrant a challenge, why would he challenge and disrupt the party?
I think he now has the numbers and a lot of those members would be encouraging him to stand for leadership. Many of those ex-Howard faithfuls.
Look what happended to Keating when he challenged Hawke and lost. Labor turned into an absolute rabble and Keating was sent to the backbench.