It's very difficult to answer your question in few words. I'm listing important reasons only:
1. Writing and publishing on hubpages is simple.
2. Layout and design is simple
3. More simple is to evaluate your work both in terms of popularity as well as monetary
4. Your work gets indexed at the earliest by the search engines.
5. Hub and Hubber score are indicators of your success as a writer
6. Your income increases with your residency period on hubpages
7. You can draw traffic to your other sites from hubpages and reverse the traffic easily from your sites to hubpages back
8. You get a platform to raise the sentiments and sufficient number of readers for your articles
9. You can convert yourself from amateur to a professional writer (literally as well as money matters)