Mainly because, as I previously stated, it is already possible to get to BWI by train, even if the service is less than ideal in some respects. It is impossible to reach IAD by train.

Also it seems that such a line would be a boon to places like Reston and Herdon, currently without Metro service. You might argue that the econmic stimulus it would generate could be as much of a curse as a blessing, but it would get quite a few cars off the road. That's something laying another gauge track to BWI on an already-served route cannot possibly accomplish as well.