Here's why:
* Tekken Tag Tournament (released in 1999) came out before Devil Jin was introduced as a character. He debuted in Tekken 3 (released in 1997).
* He was only a hidden character in Tekken 3, not playable from the start.
To play as Devil Jin, you'll need to play later Tekken games:
* Tekken 3 - You had to unlock him.
* Tekken 4 and later - He is a standard playable character.
If you are looking to play as Devil Jin, you'll want to check out Tekken 4, Tekken 5, Tekken 6, Tekken Tag Tournament 2, Tekken 7, and Tekken 8.