Even the MLB doesn’t seem to have an answer as to why baseball stitches are specifically red, but there are some educated guesses. Before the 1900s, baseballs used natural cowhide-colored stitches. Both the National and American Leagues added color shortly after the turn of the century, likely to help the batter see the ball better as it approaches. When the MLB announced the official red standard, they likely ditched black and blue thread altogether and settled on red because it was the most highly visible color already in use by both leagues.

Source: Children's Museum