I believe they all made the switch to align with the colors of the city flag.
Good Catch.
Drawn from the family coat of arms of William Pitt, the Lord of Chatham, for whom the city was named, the colors Black & Yellow have evolved into an Identity for Pittsburgh's sports teams.
Pittsburgh was officially named in 1758 for William Pitt, who as secretary of state made key decisions that helped British forces in the colonies successfully capture from the French the confluence of the Allegheny, Monongahela and Ohio rivers.
Just over 140 years later, Pittsburgh's council decided that the city flag would use the black and gold -- really more of a yellow -- from the Pitt family's coat of arms.
In 1933, a new football team adopted those colors as its own, even emblazoning Pittsburgh's crest -- also borrowed from William Pitt's coat of arms -- on its jerseys.
The only year the team's players wore different colors was in 1943, when they wore green and white during a brief merger with the Philadelphia Eagles that also saw them renamed the Steagles.
In 1948, the Pirates make the switch from their red, white and blue uniforms to black and gold, and in 1980 the Penguins followed suit.
The city's first National Hockey League (NHL) franchise, the Pittsburgh Pirates were the first to wear black and gold as their colors. The colors were adopted by founder of the Pittsburgh Steelers, Art Rooney, in 1933. In 1948, the Pittsburgh baseball Pirates switched their colors from red and blue to black and gold.