All members of our society should have equal opportunities to become scientists, irrespective of their race, gender, religion, or affiliation to other minorities. If you really need a reason for this, read the human rights. Unfortunately, this is not yet the case, neither in the US, nor in Europe. The cause for this is partly that higher education is money-wise often not favourable, and sadly in many cases there do exist correlations between belonging to a minority (e.g. immigrants) and being faced with a difficult social background. Other reasons are traditions (physics is a boy-thing), peer pressure (white and nerdy), conflicting ethical values (does the bible mention dinosaurs?), and of course prejudices against minorities (small town guys are stupid).
Since my weekend has only 48 hours, these are the points I don't want to discuss today. What irritates me about the discussion on diversity in science is the entanglement between the issues of minorities in our society, the social diversity in the community of scientists, and the diversity on the 'marketplace of ideas'. An entanglement that should be approached with caution.
