Because "Itu kucing" = That's a cat/the cat.
If you use "itu" in the first place, it's an implied form of "Itu adalah kucing" = This is cat" literally. Itu would be the "that" subject-pronoun in English, subject of "is" in "that is".
If you want to use "that" as a qualifier for "cat", you have to use it after the noun.
My cat = kucing saya (after the noun)
That cat/the cat = kucing itu (after the noun)