Billy Graham is/was a man of religious and so political prominense. HE was sought out by many with political ambitions. Very early in his career, he was to meet with a president, Eisenhower I think. He did a photo op of kneeling on the White House lawn to pray, and it came back to bite in in the a**. Since then, he's avoided overt public attention with politicians. It doesn't mean he's avoided them, he just learned not to be in the spotlight with them.
Every president has had their "spiritual advisors", and Billy has been to a number of them. Bill Clinton asked Billy, but Billy graciously declined. I would say for a few reasons, though I don't really know. 1-Bill Clinton wanted to be associated with the most well known and respected christian in America, if not the world. Billy Graham would have given Clinton legitamacy. 2-Clinton wasn't sincere in his seeking of spiritual guidance. 3-Age. Billy was feeling old, and felt others could do just as well "advise" Clinton.
As far as point 1 goes. Bill Clinton had at least 2 "spiritual advisors" that I know of, 3 if you count the Rearend Jessie Jackson, but I don't. 1-was Tony Campollo a fairly popular pastor, with some pretty liberal leanings. religously, therefore politically. And 2-Bill Hybels, pastor of the largest, or one of the largest churches in the US. Though Hybels wanted to keep the association low key, and for the most part succeded. Clinton still used the assocation to his benefit, to give him credibility.
Billy Graham is now a very old man. The fire that is left in him, he wants to use for important issues. Though politics are important, politics only affect some, and only affect the present. Billy's message affects eternity for everyone. So he chooses the most important battle to use his energy.
Why was Clinton at a Billy Graham crusade? I have no clue. I would have laughed at him and kicked him in the a** on the way out. (better yet, kicked his wife in the a** on the way out) But the Grahams are extremely gracious people. Also, why not USE Clinton for your purposes, like he uses others? What better way to draw non-christians to an event, than to have the most beloved president to non-christians, speek at the event. Knowing that BJC isn't going to say anything damaging at the event. At least nothing that can truely damage the gospel.
Besides, when the spawn of Satan is the senator of New York, why not use Satan against Satan?