Problem is, paying a 5th umpire. But also having a setup in every stadium so that he can review it.
It's easier to have one location with the most up to date equipment determining the call.
And when you think about it. There is a difference, between a guy 500 feet, looking at replays making a call, and a guy 3000 miles away, looking at replays making a call.
I would actually think the guy 3000 miles away would be a better alternative. Because if it's a 5th umpire, then that umpire may be on the field the next day making a make up call. Also, a 5th umpire would mean you are contradicting your fellow umpire. We know how that's one of those stupid unwritten rules.