[jakub_h]

I think the primary reason may have been that before the advent of stored-program digital computers, vast arrays of people with mechanical calculators were used for computational tasks. When the first digital computers came, the job was still sufficiently easy that the skills were easily transferable - the first electronic digital machines were glorified calculators. Everyone thought that making programs would be trivial so "clerical" people were used. Only when the complexity of software started increasing the scale of the problem became apparent, and a new field was born. And the newly-trained young people who came into a booming field came with a different demographics. The job description changed significantly, it's not (only!) that the same job was suddenly usurped by male applicants.