A couple of reasons.

1. The amount of player involvement in determining the story – The amount of control the players have really change it from an ARG to an interactive series.
2. It’s easier for me to explain the concept to others as an Interactive series than an ARG. Saves me like 10 explanations.
3. My involvement – Being directly involved (with this website) changes the immersion. Instead of ‘What is this crazy possibly real story?’ now becomes ‘here is my universe, come play.’