The Boston Celtics went into game six knowing they needed more offense than game five, while repeating their best defenisve performance of the series. Not only was their defense not as good, but their offense somehow managed to get even worse.
They were out of rhythm and taking bad shots, and when the open look was there, nobody was able to hit it on a regular basis. The Milwaukee Bucks were able to disrupt a lot more than the Celtics were, and they executed at a much higher level.
The Celtics have been inconsistent all year long, so this game should not be too much of a surprise.