PS. I should also add that many from multi-ethnic backgrounds do not have the residency or UK nationality requirements to hold the security clearances required too. Most aircraft types need DVs these days too - all Fast Jet and ISTAR types, and growing numbers of helos and transport aircraft. So that is another issue. Again, it will come, but having parents with non-UK passports or single-digit years of residency in the UK will stop many - as will criminal records should they be unfortunate to live in an area where such things are commonplace. Again, that is not for the Armed Forces to resolve as these are central Government policies that affect.
PPS. Just looked it up on the web. For a DV you should have resided in the United Kingdom for the 10 years immediately preceding your application but candidates with a minimum of 7 years will be considered. You must be a citizen of the United Kingdom since birth and must not hold (and must never have held) any other nationality or citizenship (including joint nationality) with the exception of dual British Irish citizens. So if you have parents that have registered you for dual citizenship as they hold foreign passports then that will cause recruitment issues.