The reason why members of the Toronto Blue Jays and other MLB players wear No. 21 on Roberto Clemente Day is to honor the legacy of Roberto Clemente, a Hall of Fame baseball player and a revered humanitarian from Puerto Rico. Clemente wore No. 21 throughout his career, and MLB has designated September 15 as Roberto Clemente Day to celebrate his achievements, character, and contributions to the community.

Many players wear No. 21 to pay tribute to Clemente's remarkable impact both on and off the field. The gesture is a way to recognize his accomplishments, including over 3,000 career hits, his role in philanthropy, and his status as a symbol of Latino heritage and resistance.

Specifically, for the Blue Jays, several players and staff wore No. 21 during their games to honor Clemente's legacy, and the team also displayed patches and insignia related to Clemente on their uniforms and gear. This is part of a league-wide observance where all MLB players and personnel may wear No. 21 or patches as a tribute on this day, which is celebrated across the league, especially in Puerto Rican and Latino communities.

In summary, players are wearing No. 21 on Roberto Clemente Day to honor and remember his extraordinary impact, as well as to celebrate Hispanic Heritage Month and promote his legacy of perseverance, excellence, and humanitarian work.