That is because it's almost impossible to maintain dyed blonde hair when you have naturally very dark brown hair. With current hair dyes you can only hope to achieve an unflattering shade of bright orange or yellow, or spend literal hours maintaining a neutral shade. Most people who dye their hair blonde are mousy midtone brown or dark blondes, because it's easier for them to maintain it.
BUT that doesn't mean your argument isn't valid, and yes, there should be more darker haired people. What I'm saying is that there's no excuse not to have blondes as well.
To clarify, I don't have a problem with a little more light hair and admittedly, it was too scarce in the trilogy. In fact, it
would be refreshing if we had a blonde squadmate for a change. But there have been three games now and in each
one blondes are few and far between. For them to appear in disproportionately large numbers in Andromeda would
be jarring. So I'm leaning toward lore in this case, even if it rests on a somewhat questionable premise.
However, I do think that a reasonable compromise between lore consistency and more blondes can be found.