A primary purpose of the Constitution is to balance power among the states. Electors may vote for one candidate from their home state, but not both. This is to make it very difficult for a President and Vice President to come from the same state.

This is also why in 2000, Dick Cheney sold his home in Texas and moved into his vacation home in Wyoming, so he could be Vice President to W. And it was a good move too, considering they won by a single electoral vote. If he had kept his Texas residency, he would not have had enough electoral votes to be Vice President, and we'd have had a gop president and dem vice president.