The Republicans met in New York City for far different reasons than what they would be meeting in Minneapolis for. The Republicans were under no illusions that they had any chance to carry New York in the election.
The convention in New York City emphasized the President's and the party's national security credentials, an issue on which the President and the party were far ahead of Kerry and the Democrats at the time.
Besides, Minnesota is much closer to being winnable by the Republicans than is New York. In 2004, the spread in New York was 18.29%, while in Minnesota the spread was only 3.48%.
Minnesota has gone from being a very strong Democratic state to nearly a swing state. This state was once among the most Democratic of states, and now is toward the middle of the pack. So it's not a bad idea for the Republicans to woo the state.
Of course, they met in Philly in 2000 and failed to carry Pennsylvania, despite it being a swing state. So I don't know how much the location of conventions really affects the voting.