There is no reason.
It's a bullshit question that we're all required to ask when working in Government agencies, because they think if they at least ask the question they can make it appear as though they're genuinely concerned about a minority group.
Nothing gets done with that data. It's collated, some statistics are generated, the statistics are discussed and sometimes published in a paper, and then the questionnaire forms get filed away forever. All it is used for is so that some people can sit around a desk and say 'Oh my of the 638 people who had this procedure, or sat this test, 4 were Aboriginal.'
They're irrelevant and you don't need a questionnaire on a form to see whether there are inadequacies in life expectancy, literacy/numeracy etc.