I have no idea about the situation in the US but could this be due to other factors? For instance, you mention that these minorities are under represented in certain colleges to begin with - a place where a lot of startups begin. Perhaps there is less internet exposure too, or at least to the startup community blogs etc.. And maybe it just comes down to a perpetuating loop situation. In any case, and I don't really know US attitudes, but I would doubt in this day and age there is a conscious effort among hackers, who are generally more liberty and equality focused anyway, to exclude people based on anything other than merit.
