Why 15? why 8? (mercy rule)

[NDblue]

I wish we would have had that 15 run rule in the McQuade's Charity SP softball tournament. 75 min time limit. I'm umping a game that after 3 innings, the winning team was ahead by 30 runs. This was 97 minutes into the game. They're required to play a minimum of 4 innings no matter the time limit. Losing team is home so they get last bat. Top of 4th inning, I started to make up outs to get the game over with. Neither coach said a thing and just nodded. Final score was 38 - 0. I couldn't help pitches that were landing if front of the plate or dang near hitting me in the chest because they were too deep. Anything that looked remotely close was called a strike.