Why did the Chicago Cubs play 2009 home game in Milwaukee?

The Chicago Cubs played a home game in Milwaukee in 2009 due to a scheduling conflict caused by the renovation of Wrigley Field. The game, originally scheduled to be played at Wrigley, was moved to Miller Park, home of the Milwaukee Brewers, to accommodate the renovations. This unusual situation allowed fans in Milwaukee to enjoy a Cubs game, while the Cubs managed to maintain their schedule despite the construction work.