Back then programming was basically applied mathematics, a field with many women. Today programming is gluing together components in order to build systems which is much more similar to engineering, a field with few women.

There are still many women among those who program mathematics (statisticians etc), just that they are usually not called programmers. Also there is much less demand for people who can program math than people who can glue together libraries and create crud apps, so even if all math programmers are included in the statistics they would get dwarfed by the app programmers.

Source of the combination mathematics and statistics being a gender balanced field, above 40% women: https://www.nsf.gov/statistics/2017/nsf17310/digest/fod-wome...

Engineering always being male dominated, around 15% women: https://www.nsf.gov/statistics/2017/nsf17310/digest/fod-wome...

Computer science gender balance getting lowered to engineering levels: https://www.nsf.gov/statistics/2017/nsf17310/digest/fod-wome...