force out cancels scored run

Question
I understand that the rule is that even if a runner crosses the plate before the third out is made, the run is not scored if the third out is a force.  But why is this the case, what is the reason for this rule?

Answer
Mark,

The thinking behind this is that you must advance a base when you get a hit in order to let someone else score.  If you don't hit the ball well enough to force runners to their next base, then you don't earn the run.

Basically, this is one of the fundamental rules of baseball.  It has been in effect from the beginning of professional baseball.

Hope this helps!

Brian