Because Santa isn't coming this year. Yup. He wants to take a holiday.

6 seconds sound clip from the The Year Without a Santa Claus movie soundboard.

You can hear this line at 00:25:16.099 in the Blu-ray version of the movie.

Quote context

[...]

- Well, it just so happened that Jingle and Jangle had the same good idea.

- When the man at the dog pound refused to let Vixen go, he suggested that they take their case to the mayor.

- Now, let me see if I got this straight. You two are elves and that dog down at the pound is a reindeer. And you wanna take him back to Santa Claus?

- At the North Pole.

- You see, Your Honor, we're here on a special mission.

- A secret mission.

- A secret mission. I should have guessed.

- Because Santa isn't coming this year. Yup. He wants to take a holiday.

- No more, please. I can't take any more. Reindeers, Santa Claus, elves...

- But we are elves. We're serious. How can we convince you?

- Oh, come on, son. What kind of fool do you think I am? You ain't got the chance of a snowball in Southtown of convincing me one-tenth of your story's on the level.

- A snowball in Southtown? What kind of chance is that?

- Not very good. It never snows here.

- Ha. Well, not in the last hundred years.

[...]