Why isn't he creating a political party rather than running for President? This must be the only-two-parties rule again? I heard once that there are more than just Dems and Reps in the US political system but I never hear much about them.
The reason why is because it's dead easy to run as one of the major parties and use their ballot line. In some states it is practically too late to try to start a new party or run as an independent; it's an incredibly time-consuming and resource-draining process. If he runs as a Democrat, he's on the ballot in all 50 states and DC. If he runs as an indy, he might be able to reach somewhere in the 30-40 state range.
Unlike the UK (which I'm assuming you're from just because that's the easiest assumption), where a few signatures and a deposit gets you on the ballot, here each state has different rules, and most states unabashedly try to keep independents or minor party candidates off the ballots with unequal laws.