According to Britannica, it sounds like it was a bit of an equality issue - Mother's day had already been invented and was gaining ground. While many think of the two days honoring parents as being driven by businesses in the business of selling cards and such, it's actually a holiday designed by average citizens a very long time ago. Either way, your favorite father deserves a box of Scottish Fix shortbread!
"Father’s Day, in the United States, holiday (third Sunday in June) to honour fathers. Credit for originating the holiday is generally given to Sonora Smart Dodd of Spokane, Washington, whose father, a Civil War veteran, raised her and her five siblings after their mother died in childbirth. She is said to have had the idea in 1909 while listening to a sermon on Mother’s Day, which at the time was becoming established as a holiday. Local religious leaders supported the idea, and the first Father’s Day was celebrated on June 19, 1910, the month of the birthday of Dodd’s father. In 1924 U.S. Pres. Calvin Coolidge gave his support to the observance, and in 1966 Pres. Lyndon B. Johnson issued a proclamation that recognized the day. It became a national holiday in 1972, when Pres. Richard Nixon signed legislation designating the third Sunday of June as Father’s Day."