Mostly becaue of its blatant disregard for much loved characters (and uneven inclusion thereof - Lando's a hero! Lando doesn't exist!), and heavy handed plotting that essentially deadened the thing that sets star wars apart from any other sort of soft-science space opera like star trek - the Jedi.
Combine that with an overarching metaplot that takes the less unique (but just as interesting) parts of the star wars universe (like its well-developed criminal underworld) and marginalizes it to the point where it may as well not even exist, and you have a series of novels so unsatisfying that they cannot be enjoyed - only suffered through for (exceptionally) brief moments of good writing (like the rogue Vong shamed one who helps Anakin and Tahiri escape).
The dogged insistence of the series' authors that the republic/rebellion never win anything made what could have been a perfectly satisfying and interesting 3 books into a couple of years of ridiculous rebel-bashing. As soon as a tide started to turn, another author would cut in and reset any gained advantage to tell a tale of more losing.
Furthermore, to insist, on top of everything else, the Nom Anor is unique as a sympathetic Star Wars villain is ridiculous. Vader, anyone? Not only does the real thing reek evil in a better way than Nom Anor ever could (or did), but his redemption actually meant something. Nom Anor's switches had nothing to do with his character - in fact, they reveal nothing so much as the inability of the myriad authors of the books to work together to establish a unified personality for any of the characters in the series. Han is an even better reflection of this than Nom Anor!
More than any other work, the NJO suffers from "Too Many Cooks in the Kitchen" Syndrome - all of the above adds up, IMHO, to taking everything that makes Star Wars enjoyable and unique, and crushing it.
Sorry I have to disagree Ankh-morpok, but I loved the expanded universe *until* the NJO. Since it began, I have been increasingly disgusted with the future of star wars. You are (unfortunately) just bearing the brunt of my frustration.