The main reason why most fundamentalists do not think that Obama is the anti-Christ is that ‘his’ identity will be hidden until the Rapture. II Thess. 2:3, 7-8. A secondary reason is that he is not the right nationality. That reason is more open to debate.
Another reason is that the anti-christ will be MUCH more slick and polished and fool way more then 52% of the population. Obama is becoming more unpopular by the day, this will not happen to the anti-christ.