Because it is part of the Dem's political strategy of distract, divide, and attack. They will try to make the opposition appear to be the lunatic fringe, i.e., birthers and deniers. It has nothing to do with Obama's policies. Rather it has to do with his citizenship and his race. The MSM will be his lap dogs in framing the issue that way in much the same way they have perpetuated the myth of manmade global warming. The overwhelming majority of Americans believe that Obama was born in Hawaii. And they are not aware of such distinctions as natural born when it comes to being born of a parent who is a citizen of another country.
Here is the Dem strategy memo.
"The memo urges Democratic candidates to force their opponents to answer a series of questions on health care, taxes and some of the favorite causes of the far right:
“Do you believe that Barack Obama is a U.S. citizen? Do you think the 10th Amendment bars Congress from issuing regulations like minimum health care coverage standards? Do you think programs like Social Security and Medicare represent socialism and should never have been created in the first place? Do you think President Obama is a socialist? Do you think America should return to a gold standard?”
"If a Republican candidate says no to any of the questions, the memo says Democrats should “make their primary opponent or conservative activists know it. This will cause them to take heat from their primary opponents and could likely provoke a flip-flop, as it already has several times with Mark Kirk in Illinois.”