Looking at it in historical context, Bush did better in 2004 than any Republican presidential candidate since 1988- and by a huge margin. Indeed, 04 Bush was, by sheer percentage alone, tied for fourth most popular Republican candidate since Eisenhower. Increasing polarization, especially since the 80s and 90s culture wars, meant there were fewer swing voters to court than in most previous elections, and new methods of reaching out to voters (the Internet was just beginning to reach the levels of participation that eventually let it become the backbone of society these scant few years later, and analytical data allowing for microtargeting of potential voters) was something the 04 Bush campaign pioneered.
The fact that Bush made such a strong showing in spite of his flaws- his 'compassionate conservatism' rubbing culture-warrior voters the wrong way, the attempts to privatize social security backfiring, his complete spurning of the isolationist wing of his party, and the sheer and simple fact that he was president in an era where there just simply were more Democrats than Republicans (as evidenced by the fact that Democrats won the popular vote in- again- literally every election since 1988 besides this one)- says that to some degree Bush was quite possibly more or less running about as close to as high as he could possibly reach short of the election being held in, say, early 2002.