First, because Barack Obama Sr. was a subject of the British Crown at the time Barack Jr. was born, Barack Sr.'s children, all of them, became subjects of the British Crown. Barack Jr. has stated this very clearly on his former "Fight the Smears" website which has since been srubbed. If you wish to read what his campaign wrote visit just about any blog covering this story and you can pull it up and read it with your own eyes.
What this means is that Barack Sr's loyalty lied with the Brits. Ann Stanley Dunham, Barack Jr's mothers, loyalties lie with the USA. When Barack was born to these two individuals he became a citizen of the US by virtue of his mother's citizenship AND a subject aka citizen of the British Colony aka Kenya.
This status of citizenship is known as "Dual" citizenship. Note: some argue that if Barack Jr was born in Kenya then his mother was to young to pass on US citizenship, hence, why folks want to see his original vaulted long-form birth certificate.
To complicate matters further, Barack Obama Jr was adopted by an Indonesian citizen named Lolo Soetoro. If he was adopted under the age of 5 he would have automatically become a citizen of Indonesia regardless of a legal adoption. Note - the US gave deference to sovereign territory laws regarding dual citizenship, ie., American and Indonesia or British and Indonesian, etc. Indonesia, at that time, did not recognize dual and only recognized Indonesian citizenship. Therefore, Barack Jr through adoption became an Indonesian citizen. No, he did not ask for this but as a minor child he was under his parents guardianship.
If Barack Obama Jr. went to Indonesia at the age of 6 he would have had to have been formally adopted by Lolo Soetoro in Indonesian court. The citizenship status, as noted above, would still apply.
Now, we already know that Barack, by birth, was a dual national, British and American, which means he can only be a "native" citizen, that is, if he was born in Hawai'i as he has stated AND if when he returned from Indonesia he applied for and received his status as a naturalized citizen. If he did not apply for naturalization then he is an illegal alien. Few question whether he didn't apply for naturalization but there is no record of it to date so it cannot be ruled out.
Our Founding Fathers made it clear in Article II of the US Constitution that the President must be NOT may be a "Natural Born Citizen" for a reason. They wanted this leader to have loyalties ONLY to the USA rather than divided loyalties such as the USA and Britian - remember these good men gave blood and treasure during the Revolutionary War to secure freedom from the British Crown. They did not want the leader of this country to be loyal to any other homeland.
The definition of a "Natural Born" citizen is two "natural born" parents of the USA. It's nothing more complicated than this. If you were to marry another natural born individual and the two of you had a child in the USA or abroad at a military base or embassy your child would be a NBC.
Barack Obama Jr., by birth, never had the citizenship status of a "natural born" child and is therefore ineligible to serve as the President of the USA according to the laws that govern our nation.
Now if you want to direct your hostility to the appropriate parties who looked the other way then please look no further than the DNC, Howard Dean, Nancy Pelosi, Harry Reid, Ted Kennedy, John Kerry and Diane Feinstein. These are the culprits who signed off on the eligibility documents required by individual state's Board of Elections. The Party is the sole determiner in vetting their candidates eligibility. No federal agent is required by law to vet said candidates.
Barack Obama Jr is by definition under our laws a "native citizen" AND NOT a "natural born citizen". This is fact as he has clearly stated himself.
