And here's why:
"Little did anyone know on December 18th 1983 the Colts had played their final game in Baltimore. Almost after the season ending win at Memorial Stadium; Colts owner Robert Irsay began talking to other cities about moving the club. On February 13th he toured the Hoosier Dome in Indianapolis. The Colts also spoke to officials in Phoenix, as the Maryland legislature tried to use eminent domain laws to force the franchise to remain in Maryland. On March 28th Phoenix dropped out of the bidding, at Irsay called up officials in Indiana saying he was on the way. However, he kept the move secret until just after midnight when by cover of darkness on a cold snowy night he used Mayflower moving trucks to pack up the Colts offices, and relocate them in Indianapolis. By the time anyone realized what had happened it was too late to stop. The NFL feeling they could not win in court did not fight the move, as Maryland put up a futile fight before the Colts agreed to support Baltimore getting an expansion team. The city of Baltimore would fail in several attempts at landing a NFL franchise, and even entered the CFL, before landing a new NFL franchise named the Ravens in 1996. However, the Irsay family would not relinquish the name Colts, as they once promised. Many Colt legends were angered by the move including legendary QB Johnny Unitas who refused to acknowledge the franchise for the rest of his life."
From the Sports Encyclopedia.