Personally? Because he's largely been rhetoric over policy and what policies he has introduced aren't differentiated enough to make me want to support him over some of the other candidates who, yes, do in fact have ample experience (namely, Biden, Dodd, and Clinton). I don't find his claims to having good judgement, namely because he was against the Iraq war from the start, to be unique or compelling enough to single-handedly gloss over all other aspects that a presidency entail. Nor do I find his counterargument that experience is potentially a detriment to the country's welfare; the Bush administration is, arguably, more of an exception than the rule.
PhoenixDark: Obama isn't pursuing the far left; that's Kucinich and Gravel's respective jobs. He has/had popularity with a fair chunk of independents and Republicans (although he may have lost that to Clinton in recent months), but that alone should indicate that he's pursuing more of a centrist set of policies, at least insofar as appeal goes, than you suggest.
