The reason is simple...
Just like a game may go OVER in 5 innings but gets called, the same can happen to someone betting the UNDER...
Imagine, how they would feel if the TOTAL was 12 on a Colorado Rockie game and the game was called after 8 innings with the score being 0-0 at the time....
That's why totals have to go all the way...
To be fair to over betters as well as Under betters!