Men specialized in hardware while software development was seen as an exciting alternative to secretarial work. In 1967, Cosmopolitan published an article titled The Computer Girls, encouraging young women to pursue careers in computer science. So the curve went up, and continued to do so up until 1984. That’s when personal computers appeared.
When Apple released the Macintosh 128K and the Commodore 64 was introduced to the market, they were presented as toys. And, as toys were gendered, they were targeted at boys. We can look at advertisements from that time and quickly find a pattern: fathers and sons, young men, even one where a man is being undressed by two women with the motto Two bytes are better than one. It’s more evident with the ads for computer games; if women appear, they do so sexualized and half-naked. Not that appealing for young girls, one could imagine.