This is an issue that's been bugging me for a long time...and a big part of the reason I've lost interest in the Star Wars universe for the most part.
You ask why so many interesting characters get created, and then are completely ignored outside of whatever publication they were introduced in.
To sum it up, most authors don't give a rip about anyone else's material. They'd rather make their own crud than use preexisting elements. The best examples of this would be the work of Timothy Zahn and Troy Denning, for those of you familiar with their work. Every story they write features elements that aren't found anywhere else in the canon, not even mentioned offhandedly.
Granted, Zahns work is quite more permanent than most other authors, everyone's familiar with the Chiss and his pet character, Thrawn. But every novel he writes is completely dedicated to the subjects he's created, and he pretty much ignores anything else out there (ie, Hand of Thrawn duology features a conflict between two races unmentioned anywhere else in canon, that throws the entire galaxy on the brink of civil war). Though again, Zahn is talented enough to get away with that, and a lot of his work is central to the EU universe.
But I would stab my eye out rather than sitting through more of Troy Denning's insufferable garbage. If you want to know who's responsible for the dissappearance of quality characters and elements in the SW universe, he's a big part of that problem.
Simply because he does exactly what Zahn does, but lacks the writing talent or creativity to pull it off. And sadly, he's handed commission after commision for pumping out more of his trash. Killik Twilight, the shallowly idiotic Squibs he seems detirmined to make main characters, and other annoyingly uninspired elements are the centerpiece of his work.
The crux of the problem is, authors are just too pompous to use other people's material, and feel compelled to make their own stuff--the end result is a universe full of one-time, shallow characters that are ignored even if they're a good concept that could work amazingly well given the proper exposure (namely BalanceSheet, though I admit I am a little biased ^_^).
"If you don't want me to eat you.....SAY SOMETHING."