His personal scandals derailed him in 2000, not 2008 by which time they were ancient news.
No, Giuliani's problem was the same as Ed Muskie's in 1972: a roof without a foundation. Giuliani had no base within the GOP, was too socially liberal (John McCain, stem cells aside, is a social conservative) and his fiscal/national security hawk constituency had already been claimed by McCain. Furthermore he was either unwilling or unable to engage in the sort of retail politics that wins Iowa and New Hampshire, with a poor campaign strategy. For the general election,
Salon explains what would happen quite well.
Even if Giuliani wins New Hampshire and Florida, he needs to win Iowa and/or SC. Both of which are dominated by social conservatives. Then there's the rest of the South, without which a Republican nomination cannot be won. The math is simply not there.