Because its unprecedented in American history that a national campaign was waged disputing the citizenship of a sitting POTUS that was so effective people believed it and coupled with the history of the Jim Crow grandfather clause where black voters were required to submit documentation that their grandfathers were voters shortly after slavery had ended, many blacks felt there was an unspoken racial component. I personally think for most birthers it was not a race issue but rather they honestly had been unduly influenced by aggressive and effective propaganda tactics employed by people whom they trusted.
That's generally my take on it.
Were some people motivated by racism? Absolutely.
Were some people motivated by other reasons, and then went down that path because of racism? Absolutely.
But I think by and large, the base motivation of the birther movement that was most wide spread was political in nature...not racial. Every president tends to have the opposition latch onto SOME issue that they use to try and delegitimize and attack. While there were obviously some within that group who went bought in because of dislike of black people, I would wager most bought in because of a dislike for Democrats.
I think the argument for where race played a larger factor was the method, or the substance, used to launch the attacks. Or put another way...I think race played a bigger roll in using the "birth certificate" as the topic of attack than it did in the existance of an attack in the first place.
However, even then, I think a lot of factors went into it. For some, it was a racial component. For others it was more of a xenophobic notion of someone "not from here" and "foreign". For others it was a religious fears and issues that caused them to buy in. While it's true that Barack Obama's race is unique amongst Presidents, those who seek to see Racism everywhere often fail to notice other unique aspects of him that helped play into the crafting of the "birther" movement as a potential attack.
No President in recent memory spent near half a decade of their lives living outside of the country. No president in recent memory attended a school predominantly populated by follows of a religion other than Judaism or Christianity, nor included teachings about said religion. No president in recent memory had a parent who was a follower of a religion other than Judaism or Christianity. No President in recent memory had a name which was no where to be found on the list top 200 child names of that decade.
All of those things, along with race, helped to create a situation where such a narriative could be crafted and created to plant enough doubt in the minds of those with a basline motivation (political) to want to believe regardless of what their "trigger" point may be (race, xenophobia, religion, "otherness", etc). I would wager if the Democratic President was white and named Jacques François, had lived in France for nearly 5 years, and had the same mild religious ties in the background as Obama had that we would've seen something similar develop...albiet at a slightly lower verocity because the race factor wouldn't be another layer added on top of it.
Why do candidates often attempt to seem like a guy "you could have a beer with?" Why is it that all other things being equal between two candidates, some Christians would vote for the Christian over the Jew/Athiest/Muslim? Or the minority would voet for the Minority over the non minority candidate? Or the small business owner would vote for a guy who grew up working his way up as opposed to a guy born into a well off family? It's because people want a politician they feel can relate to the struggles and issues facing them, and seeing things that are "familiar" or similar to ones self often imparts that impression.
This is a bit long winded but I guess what I'm saying is in many ways I agree with you. I understand fully why many black voters FELT that it was largely rooted in racism. I think there are people within the population of birthers whose participation is influenced by the Presidents race to varying degrees, and I do think there are some out and out racist individuals within that population. But I think the most wide spread common root motivation for the birther notion is one that isn't based upon race, and I think there are a multitude of factors not directly tied to race that go into why that particular method of attack was what came to rise.
