Because electing Kerry will lulls the Democratic party mainstream majority into thinking that true change has taken place when it hasn't, and take the wind out of the sails of the movement.

It is called an appeasement strategy, Nerd. Here is your Merriam Webster definition of it:

Main Entry: ap·pease
Pronunciation: &-'pEz
Function: transitive verb
Inflected Form(s): ap·peased; ap·peas·ing
Etymology: Middle English appesen, from Middle French apaisier, from a- (from Latin ad-) + pais peace -- more at PEACE
1 : to bring to a state of peace or quiet : CALM
2 : to cause to subside : ALLAY
3 : PACIFY, CONCILIATE; especially : to buy off (an aggressor) by concessions usually at the sacrifice of principles
synonym see PACIFY

The result of Kerry losing, OTOH, will be to energize those people who will otherwise be appeased if he wins. When people are energized to act, they become better informed, do more to bring about social change...

I can cite several examples, actually. I just read them in an article that I cite in this very thread. Here you go:

From Todd Chretien's article linked to above:

"The great immigrant revolutionary, abolitionist and supporter of women's rights, Thomas Paine, made the point in 1776 that in order to win any meaningful battle, it is necessary not only to fight when it is easy. It is necessary to fight, and in fact, it is especially important to fight when all "pragmatic" opinion counsels compromise, retreat and surrender. Had Washington's army sued for peace in 1776 at Valley Forge then the world's first representative democracy would never have been born.

Visionary abolitionist Frederick Douglass advised John Brown to abort his ill-fated raid on Harpers Ferry, not because he opposed the rebellion, but because he believed it could not succeed in its tactics. However, when John Brown was executed by the slave power, Douglass lauded him as the "man who started the war that ended slavery."

In 1937, Congress of Industrial Organization union leader John Lewis dared the government to break the auto sit-down strikes and "shoot him first." The auto bosses and Roosevelt backed down and we can thank the Flint rebels for the remnants of unions we still have today.

Rosa Parks refused to give up her bus seat to a white man, touching off a direct action movement that bucked those who advised to let the apartheid courts work with "all deliberate speed." The racist backlash was intense and led to the deaths, beatings and jailings of thousands of young Black and white freedom fighters. But Jim Crow died as well.

Any serious consideration of American history shows that Thomas Paine was right. Independence, abolition, unions, civil rights, suffrage, abortion, Stonewall. All great rebellions and reforms came into being because the minority who advocated "unreasonable" demands refused to disorganize their forces under the pressure of majority opinion. Instead, they held to their principles, gathered their forces, weathered the storm and showed friend and foe alike that "truth and not lies are the motor force of history."