Well it was still newly-popular when he was approached for it. He said he thought it was a silly idea at the time, but regrets his decision now. Ninja Turtles had just gotten popular 1 year before production began on this movie (1987, 1988).
I hung out with him for a day when I volunteered to help with his house campaign a while back. He told this story to all the volunteers.
Crazy coincidence:
Hung Cao was the original casting choice for Donatello in the 1990 Ninja Turtles movie, but he turned it down.
And Tim Kaine is often compared to The Penguin from the 1989 Batman movie.
So we have the hero fighting the villain from two popular movies made within the same year.
In real life.
It's gotten worse. This is at the beginning of their sign up process:
Haha, "Donald-tello".