It was raining out and wasn't supposed to stop until somewhere around 8:00PM. So rather than wait close to 5 1/2 hours in a rain delay to finish off two innings in a game that would then have necessitated the calling of the second game early due to time constraints, both teams agreed that the smart thing was just to call the first game done with Boston having a 9 run lead and trying to get the whole second game in.
This two game set is the only time Minnesota is in Boston all year and rather than having to stay another day to get the second game in by waiting forever to finish off the last two innings of a game that was a complete blowout and then possibly playing the second game on Thursday, the teams decided this was the way to go along with the umps.
I think it was a smart decision on everyone's part.