Several reasons really.
To name a few:
1. I am my own worst critic, so I rarely deem work good enough for the outside world.
2. The fan fiction that I write is a co-production, so I would never post it anywhere without talking it through first.
3. From what I have heard fanfiction.net is not really active anymore.
4. My fanfiction focuses on a single anime series, and since there are several fan groups of that anime on DA, I figured that keeping it on DA would make it easier to find an audience willing to read.
So, a plethora of reasons