It's just because he says he's Santa Claus.
4 seconds sound clip from the Miracle on 34th Street (1947) movie soundboard.
You can hear this line at 01:22:34 in the Blu-ray version of the movie.
Quote context
[...]
- Not at this time, Your Honor. I ask for an adjournment until tomorrow.
- The court stands adjourned till tomorrow afternoon at 3:00.
- There's a way. There's got to be.
- Well, it's hard to explain. You see, they're having, well, sort of a trial about him.
- Oh, no, no. It isn't that kind of a trial.
- It's just because he says he's Santa Claus.
- But I've got a feeling he is Santa Claus, Mother.
- Some people don't believe that. That's why they're having this...
- But he's so kind and nice and jolly. He's not like anyone else. He must be Santa.
- I think perhaps you're right, Susie.
[...]