Here's a list of reasons:
1) The price. To bring these guys to write scripts, it's gonna cost a fortune.
2) They're creative writers (writing books), not scriptwriters (writing scripts for movies). It's different.
3) Most if not all of the stories come from Lucas. So, even if Zahn, Stackpole, or Allston is hired, they'll still need to abide Lucas's will.
4) It'll take them more time to write those stories. I guess in the TCW series the scripts are written rather fast, like in a conveyor belt. The writers like to edit and edit, and edit until the story is good enough (not perfect, since it's impossible, but good enough to be a hit).
5) They can't do both - writing books and writing scripts (since the scripts would take a lot of their time).

I'm almost sure you can think of more reasons.

Anyway, I won't mind if at least one of these writers joins the TCW team. Not at all.

Whilst I agree on some points, I don't think that they are reasons so much as excuses. If they had any intention of improving the show, they NEED to be willing to spend the extra money and spend the time getting the scripts right. I mainly think the reason they aren't/won't be brought in is because of George.

Oh and I missed Luceno off my list of good authors, he's a good choice for fixing up the political episodes imo.

General Un-Co,
                         Signing off