Because he is still a creep. Plus this gives Hastert more credibility - he said, "Look, we know about your tendencies. This is bad. The Leadership will not support you. It is best to resign." (100% speculation)
Lando 
Ya' know, it's great news for our side but something still greatly troubles me.....
If it was all staged and fake, why did Foley resign so quickly before the bulk of the allegations were made public?
Doesn't add up.....
Because Rove ordered him to.
Foley should of just married his 16 year old cousin like ELVIS....he'd be revered!!!
Also, who passed the IMs to Soros?
He was embarrassed by "pranksters" who knew he was gay. He was a closeted homosexual outted by pranksters. Hell, my father and mother didn't know Foley was gay. Some people can hide it so well, but I had a suspicion he was gay. But to pull a prank on anyone, especially of this magnitude by making fun of how they talk or walk is low rent.
I grew up being made fun of how I walked because I have cerebal palsy.
Foley was outted by a bunch of punks and shamed by the gay community and called creep and pervert by total strangers.
How would you react?
Because he is still a creep. Plus this gives Hastert more credibility - he said, "Look, we know about your tendencies. This is bad. The Leadership will not support you. It is best to resign." (100% speculation)
Lando
Because he felt humiliated? ashamed? Maybe he allowed the little pranksters to blackmail him? Allowing yourself to be blackmailed is serious; more serious than instant messaging.