Because it is so amazingly easy to make fun of.

... consisting of abuse of language, bad storytelling and a clear case of the author's own agenda overshadowing any kind of canon love.

When I'm thinking something along the lines of 'wow, this author was in such a hurry to post a story about slash/romance/kickass/whatever, that he/she didn't have time to actually come up with any kind of plot or bother to read the text even once to catch the obvious typos' then I think it is so bad as to be hilarious. That is, once I have gotten over the initial shock and have stopped weeping in a corner. ^^

The best bad fanfic I have ever read is 'legolas by laura' and Cele*****, although technically I didn't read the last one, only the MSTing.

I *was* going to say My Immortal, but ITS MY LIFE! made me laugh much harder than MI did.

"robot ball/human baby" indeed. As well as GLaDOS being all goth and emo and trying to commit "sewised." :D

To me, a bad fanfic that is So Bad It's Good may be that way for a number of reasons. They usually have to do with the writing itself.

At the moment I'm following a particular badfic with many misused words. I have counted two instances where the word defiantly was used instead of definitely.

Sometimes the syntax makes a story hilarious. Misplaced modifiers and sloppy sentence structure can make the characters appear to be doing some incredibly absurd things.

And then there are unintentionally funny descriptions, such as "long white ankle hair".

I favour "legolas by laura" and "Forbiden Fruit" - both on the PPC Wiki. Pretty sure they're both trolls, but they're so funny I don't care.

Oh, I almost forgot; Quarter-Life: Halfway to Destruction!

http://www.fanfiction.net/s/6807247/1/

Mega Man At Mc Donalds is one of my many...

Also, http://www.fanfiction.net/s/4418854/1/LuigiandPeasleysSummerBlossom

There are many, many, reasons why I subscribe to the idea that it's a parody, but even if it isn't, it's still one of the funniest fics I've read.