It's was organized as a part of "Gay & Lesbian Pride Week" by Orlando Gay Activist Doug Swallow in 1991. After a while it took on it's own life and Gay & Lesbians and their friends from around the country now come to Disney on the first weekend in June.
June is historically the month to celebrate and remember the "Stonewall Riots" in NYC in 1969. There are many many events all around the country that celebrate and remember those people that put their lives at risk for our freedom.