A throwaway line in io9's review of TMNT (2014) is just one of the many data points on its chart of why the movie fails. But it struck me as imparting something important about why Hollywood is often so bad at this kind of thing.
"[In the new movie] the Turtles are apparently embarrassed to say 'Cowabunga,' and apologize profusely before actually saying it."
The movie-makers took something unselfconsciously goofy and funny and made it anxious and answerable to social norms. They were embarrassed by it, by the turtles' original goofiness. It made them cringe.
Knowing enough about the workings of the magic they buy, perhaps they feared complete removal? So they fixed it by making it superficial and ironic, in the hope that it would come across as a winking in-joke rather than, say, mocking the joy of perpetual, idealized adolescence to better reflect the nervous moderation of adulthood.
I guess it'll make a ton of money on the strength of relentless marketing, though, so there's that.