Hillary Clinton had also been scheduled to appear at the rally, but the former presidential candidate dropped out after the announcement that Palin would be attending. Clinton suggested at the time that the rally had effectively become a political event.
"In order to keep the focus on Iranian threats and to ensure that this critical message not be obscured, the organizers of the rally have decided not to have any American political personalities appear," the Stop Iran Now coalition said in a statement.
Ben Smith writes that while all politicians have been disinvited from the rally, "the move was about Palin." More here.
UPDATE: John McCain has released a statement criticizing Barack Obama for not joining the rally before Palin was asked not to attend. After Clinton dropped out of the event, the McCain camp pressed Obama to take her place. The Obama campaign offered to send Congressman Robert Wexler.
"Governor Palin was pleased to accept an invitation to address this rally and show her resolve on this grave national security issue, regrettably that invitation has since been withdrawn under pressure from Democratic partisans," McCain said. "We stand shoulder to shoulder with Republicans, Democrats and independents alike to oppose Ahmadinejad's goal of a nuclear armed Iran. Senator Obama's campaign had the opportunity to join us. Senator Obama chose politics rather than the national interest."