From that map it looks as if the Dems were just as strong in most urban areas in the state in 2004 as in 1996 - rural areas are radically different.
I'd have to look at the figures - I doubt it's really quite that extreme.
The difference is Ross Perot. However you interpret his influence on the election, 1996 and 2004 are not comparable becuase of him. 3 way elections (1948, 1968, 1992, 1996) are strange events.
The difference is Bill Clinton v. John Kerry. Clinton played very well in rural areas, at least for a Democrat; he was able to connect to alot of people. Kerry wasn't and got absolutely destroyed in rural areas, which is what probably cost him Ohio. It's going to be hard for Democrats to win in the furture without getting any support from the grasslands. Not impossible, but difficult.