It's a bunch of factors. 1. Perception of incumbent presidents tilts negative. Bill Clinton is considered a hugely popular president today but had his low points while in office. Same could be said of a Reagan. 2. Polarization of Republicans and Democrats. Republicans will be biased towards him. 3. American morale is still climbing out of a low point and feels generally negative towards all its leaders. I don't know if you're outside of the USA but you're asking a regular question to come out of the global community. Obama is generally liked outside of the USA. Part of that is because of how hated George W. Bush was. It's also that the global community (speaking generally) see the Democrats as better managers of foreign policy. While history is yet to write itself I do think that once he's left office for a few years Obama's popularity is going to sky rocket.