Claudio Pasqualin, who doesn't represent Del Piero but is close to the player, is quoted by
TMW as saying "With respect and remembering the Heysel tragedy, he said no."
The Italian football legend was 11 at the time of the Heysel disaster and a Juventus supporter, he's a legend at the club and didn't want to upset any sections supporters by joining Liverpool. Thirty nine people died at the stadium and it created wounds between the two giant clubs, some of which will never be healed, and Del Piero didn't want the danger of that tainting his legacy.