Why aren't we seeing 9th Doctor after he regenerated into the 10th?

We see past Doctors frequently in Doctor Who. The 10th Doctor has shown himself lots of times after being relived of his duties. Excluding The Day of The Doctor, we've also seen the 5th Doctor (Peter Davison), the 8th Doctor (Paul McGann) and the 11th Doctor (Matt Smith) after they were out of service.

Why doesn't the 9th Doctor (Christopher Eccleston) show up? I am asking from out-of-universe perspective of course.