The ACM Turing Award represents the highest honor in the computing research community, the closest we have to a Nobel prize in computer science. In this centenary celebration of Alan Turing, it seems obvious that the award should be named after him.
But back in 1966 when Alan Perlis won the first Turing award, Turing's influence on the field was far less obvious. Turing's model had only a few years earlier found its way into computer science literature, still dominated by automata, parsing and circuits. In 1966, what is now the Foundations of Computer Science conference was just renamed Switching and Automata Theory from Switching Circuit Theory and Logical Design.
So why name the award after Turing back then? This question came up during one of the panel sessions at the ACM Turing Celebration where there were some vague guesses. I asked around during the breaks and nobody seemed to know. I even put the question to Turing biographer Andrew Hodges in Cambridge and he similarly didn't know.
The Knuth prize aside, most awards are named after people who have passed away. Back in 1966 there weren't very many dead computer scientists. Even among Turing's generation, Church, Post, Kleene, even Gödel were alive and kicking. Turing would have only been 54 had circumstances been different. John von Neumann died young from cancer in 1957 but wasn't thought of primarily as a computer scientist. There were also Charles Babbage and Ada Lovelace but I'm not sure they were in people's minds back then.
I'm guessing that Turing was just the default choice for the name of the award and the ACM just got extraordinarily lucky for picking the right person in spite of themselves.