Enigma

Last night I watched the 2001 movie Enigma. An entertaining WWII spy movie about Bletchley Park and the Enigma code. Unfortunately although a lot of factual details were correct, particularly with regard to the way the Enigma code was cracked using cribs and Bombe machines, the lead character, based on Alan Turing was totally wrong. They cast him as a working class heterosexual Mancunian; whereas of course Turing was a middle class southerner who was very gay. I found the experience of watching the film quite frustrating. Why did they feel the need to rewrite Turing in this way?