There are several reasons why turtles where chosen:
1. There are many of them, A-Z
2. We like turtles
3. It's an allusion to the "world turtle" [1], as well as the Discworld turtle
4. It's turtles all the way down [2]
5. LOGO Turtle, the first "robot" that many of us programmed
6. Turtle power!
[1]: http://en.wikipedia.org/wiki/World_Turtle
[2]: http://en.wikipedia.org/wiki/Turtles_all_the_way_down