Because when he was in his first year of coaching, some of the players tried an old joke on him. They told him that players and coaching all get a free turkey at this certain deli in town, that all he had to do is sign up. He knew they were screwing with him, so he said "what do I look like, Charlie the Tuna?" (you know the starkist mascot). From there on out, he has been known in the league as Big Tuna.
It's not a funny story, nor does it make any sense to me, but it's the truth.