It's because you edited your answer more than 8 times. According to the revision history, which is the link just above your name, at the bottom-right corner of your answer, you edited it 12 times. For more in-depth information about the Community Wiki feature, please see the FAQ.
How does a post become a Community Wiki post?
There are several ways a question or answer can enter community wiki mode, and most of these ways will occur automatically based on the rules of the system.
Posts enter community wiki mode when:
- The body of the post has been edited six (6) times by at least four (4) different people.
- The post has been edited eight (8) times by the original owner.
- The post's author checks the community wiki checkbox when composing the question or answer. Note that this checkbox is only available to users with a reputation of 15 or more.
- The post is edited by its original author, who when doing so opts to check the community wiki.
- An answer posted to a Community Wiki question will also be Community Wiki. Note that when a question is made CW after answers have already been posted, the existing answers are not normally converted to CW automatically.
- The question generates more than 30 answers (15 on Super User). In this case, the question and all answers will enter community mode, as will any future answers.
- A moderator has reason to believe that the question serves better in community wiki mode.
* note that the rules outlined above are based on observations; the actual behavior of SO is subject to change without notice, and may well have done so since this was last updated...