I suppose the fact that the birth certificate was not readily available to begin with is why people got caught up into it. Not sure.
Then, when it was presented, they called it a forgery, but having the doc should have settled it. Before it was presented, however, is when it all took off. People will assume things that fit their desires. I am sure many thought Dan Rather was exactly right when he reported the Bush/National Guard stuff that also turned out to be forged.
I think hope that their "enemy" had issues, fueled both the birther movement and the Bush/Killian stuff.
Since when was it standard practice for politicians to publish birth certificates?
Why even question Obama's birth?
And a big part of the birther movement actually came after the certificate was released...