it's an anomaly. A perfect storm of elements that came together to make something that truly catches the essence of TMNT as a whole.
It's the dingy and dangerous aesthetic of New York, it's the perfect casting of of folks like Judith Hoag and Elias Koteas, it's Jim Henson's amazing puppetry that brought the Turtles to life (Plus the fact that this was the last movie Jim Henson worked on), it's Steve Barron's directing staying true to the original TMNT source (but still keeping the elements of the cartoon that helped people connect to the Turtles), it's the score, it's the sincerity, it's the overarching theme of fathers and sons, it's the fact that it came out in the exact right time period to be able to capture the world the Turtle's were born into.
It's a movie that will never be replicated because it's actually impossible to. Cowabunga!!!!