Here's an interesting link that might explain why a large number of folks support Bush. It's a poll comparing the perceptions voters have of Iraq, the war on terror, the world's opinion of Iraq, etc. grouped by Bush supporters and Kerry supporters.
http://www.pipa.org/OnlineReports/Pres_Election_04/Report10_21_04.pdf
72% of Bush supporters believe Iraq had WMD or a WMD program prior to the US invasion. 26% of Kerry supporters think this.
57% of Bush supporters think the Duelfer report concluded Iraq had WMD or a WMD program. 23% of Kerry supporters think this.
75% of Bush supporters believe Iraq supported Al-Qaeda, with 20% believing Iraq was directly involved in 9/11. 30% of Kerry supporters believe Iraq supported Al-Qaeda and 8% think Iraq was directly involved in 9/11.
57% of Bush supporters think the 9/11 Commission said Iraq supported Al-Qaeda. 27% of Kerry supporters believe this.
58% of Bush supporters believe the US should not have gone to war if Iraq didn't have WMD or didn't support Al-Qaeda, while 61% believe Bush would not have gone to war without evidence of the above. 92% of Kerry supporters believe the US should not have gone to war without evidence of WMD or support for terrorism, but 83% believe Bush would have gone to war, evidence or no evidence.
31% of Bush supporters believe the world at large oppose the US invasion of Iraq. 44% believe the Arab community oppose the US 'war on terrorism'. 74% of Kerry supporters believe the world opposes the US invasion and 75% believe the Arab world opposes the US war on terrorism.
69% believe Bush supports the Comprehensive Test Ban Treaty, 72% believe he supports the treaty banning land mines, 53% believe he supports an International Criminal Court, and 51% believe he supports the Kyoto treaty.
Of the issues polled, the majority of Bush supporters correctly identified his position on only two foreign policy issues: Defense spending (57%) and who should take the lead in Iraqi reconstruction (70%).
Only 43% of Kerry supporters correctly identified Kerry's position on defense spending, but at least 65% of Kerry supporters correctly identified Kerry's position on all the other foreign policy issues polled.
Summary: You can probably count on about 25% of people to be pretty ignorant of the world's events. Bush supporters exceed this. Over half of Bush supporters are ignorant about what goes on in the world at large. In fact, over half of them don't even understand what Bush stands for (of course, to be fair, Bush has not spent that much time in explaining his own positions - it's been more beneficial to call Kerry names).
Edit: Actually, averaging the poll results, I guess you could conclude about 40% of the US population is pretty clueless about world affairs, but that's just to depressing to even believe.