The show ran way too long, suffered from extraordinarily poor long-term plot development and writing, and cliched itself after about the 5th season. The new characters of last season were meant to give the show a "next generation," but the characters are not as interesting (or as attractive) as the originals.
I got tired of the repetitive episodes that gave the same conspiracy information halfway through season 6. The series, while great in its inception and novelty, just fizzled because Chris Carter was completely incapable of developping a convincing long-term plotline that everybody could follow. Many viewers (including myself) finally threw in the towel after hearing the "all your questions are answered, the mysteries are revealed....." crap over and over again.
Unlike Babylon 5, The X-Files didn't have the plot and airing time worked out in the beginning. Carter milked it as long as he could by teasing th audience, he just didn't figure out that you need some different teases to maintain interest.
End game, Carter killed his own show because he wanted to milk it (and make a fortune, which I can't blame him for) for longer than the audience was willing to stand.
/geek mode off