Because I've always found the joystick sloppy, wildly inacurate and needlessly large... Additionaly, getting a bit of hardware for one game type (let alone ONE game) seems like such a waste. I remember when I got a joystick for X-wing Vs Tie Fighter... Used it for that game, and then it gathered dust.
Still, if Lucas Arts put out "Tie Fighter 2: This time it's personal", and said it was joystick only, I'd prolly still pick it up and get a cheap'o joystick from some place.
Now stop questioning me and divert stored energy from my lasers to my shields!!