Money is what this ultimately comes down to, Marvel don't want to take the risk at this moment in time, they definitely don't want their first minority superhero film to flop or it to turnout like Blade 3.
The problem is that there haven't been a lot of films of this type, that haven't had a "star", I cant think of any big budget film with a black lead that hasn't starred Will Smith, I'm talking about a budget that's $150M+ so that will make the execs even more cautious to finance and make the film in addition to the large black supporting cast(do they do a Django and have the character surrounded by whites and a few black supporting characters to be "safe", you're already looking at controversy there, you can get away with that in a teamup film but in a solo?).Thor was filled with Oscar winning and flavour of the month actors and based on a mythological character. Marvel aren't interested in "small", "Standalone stories", "low budget" films, that's what the one shots are there for. Guardians of The Galaxy is simply another movie with a "white" lead which is the norm of the majority Hollywood movies, especially the big budget one's, that's as safe as you can get. It also comes down to other things like will there be enough toy units and mini sets to sell?, will enough toy units be sold?, will the merchanidse sales be high?, will tie-in promoters and investors be attracted to the character and film and so on. It's the same with female superheroes films.
Also Hollywood is run by a bunch of people who see the white demorgraphic as their main audience to cater to both domestically and Internationally, after domestic the money comes in from Europe(Russia, France, Germany, U.K, Italy) the demo's are primarily "white" then followed by Asia and Latin America. Furthermore the Hollywood movies that make $$$$$$ in Asia and Latin America tend to either be big budget sfx driven blockbusters e.g. Avatar, Transformers, The Avengers, Iron Man etc, something "black" movies aren't or led by a world "star" e.g. Depp, Smith, Di Caprio, Pitt etc.