Because it's difficult to identify aspects of computer technology that are unique to the field and not present in other fields in which women have far better representation. It's simply difficult to come up with a simple explanation for why women would be just fine with graduate-level math, molecular biology, astronomy, or statistics work, but not with computers --- especially given the fact that computer work dominates most of those fields, too.

And the explanation would need to be simple, because it competes with another explanation, which is that computer technology is systemically biased against women, the way every other profession was before those professions took explicit steps to correct disparity.