My top reasons:
5. The awesome set-up for creating hubs with photos, videos, etc.
4. Really cool folk here;
3. HP actually reads suggestions from its members and make the effort to improve based on that feedback;
2. I love how HP respects its members and how that respect is reflected back by the happy Hubbers;
1. A lot of the other popular writing sites have a "Permanent Irrevocable Perpetual Right" clause into their Terms of Service. That one little clause allows sites like Xomba to keep your articles on their site and earning THEM money, even after you've closed your account. I think that's a dirty trick to play on people and and have listed several of the popular sites that do this in my article titled, "Protecting Your Writing". I have a lot of love for HP simply because they don't have that clause.