It is because they were so awful that I like to pretend that they don't exist. The Matrix was such a good film, great ideas, good story, incredible special effects and action. The second and third films might have had the same effects and action, but apart from that they were just crap. I sat there through most of the second and third films bored out of my mind. I came very close to walking out of #3 and I have never walked out of a film in my life.
I have the same issue with Pirates of the Caribbean, #1 was so good that I at least expected #2 to be watchable fun. But it wasn't and there is no way that you could convince me to waste my time watching #3.
I'm normally quite good at avoiding watching crappy films (I have a mate who just goes to see any old crap), but I so wanted Matrix #3 to be good that I went to see it even knowing how awful #2 had been. Never again...
...and breathe, rant over