Why he left unstated.
The most obvious reason would be that the other two network chat & chews are an hour long (ABC's This Week and NBC's Meet The Press).
I have no idea why CBS let it drag on this long. It's a change they should have made in the 90s. While ABC and NBC were offering a full houw, Face The Nation really couldn't compete because if you had to choose between, say, This Week and Face The Nation, one is a half-hour, the other is an hour, you will go with the hour long show in most cases.
It will be interesting to see what the ratings look like in a few weeks when everyone is used to all three shows being an hour long.
"Iraq snapshot" (The Common Ills):