Number of innings pitched to receive a win or loss

Question
Brian,

Why is it that a strating pitcher must pitch a minmum of 5 innings to quailify for a win when a starting pitcher can receive a loss in a rain shortened game when he is the picher for a visiting team when only 4 1/2 innings are complete with the home team leading (official game).

Answer
Scott,

Because that's what someone decided the minimum was...

In the event of a rain shortened game (a 5 inning one), the starter can pitch only three innings and be eligible for the win.  In all other cases, the rulebook states that he must pitch 5 innings.

Hope this helps!

Brian