Because they designed the entire selection procedure. I don't think there's any reason to think they deliberately designed it to favour homogeneity, and I am absolutely not ascribing malicious intent to their approach, but when designing the selection process they gave little/no thought to diversity, and ended up with a panel overwhelmingly full of white men.
Geoff Keighley is not the global head of the patriarchy, he is just a man who is very much part of the establishment, for whom diversity is way way down the list of priorities when assembling a judging panel for his awards ceremony/advertising party.
He is framing TGA as a celebration of gaming's cultural significance. His words. It would seem that he does not consider having diverse representation within TGA as being important to the cultural significance of the medium. I disagree with this, and it seems at least a couple of the judges he chose disagree with this too.
Edit: Also after getting the shortlist of candidates back from the publications, they seemingly had no thought process before publishing them. Many people would have thought "Hmm. This list is overwhelmingly full of white men, perhaps I should contact one of the huge number of qualified women or non-white people in the field that I did not invite to participate yet". Even if somehow they only know white men (which I entirely reject), there's any number of ways to solicit suggestions for people who could expand the diversity of the panel.
No one put a gun to Keighley's head and forced him to accept the initial shortlist as the only possible range of judges.