I can share a bit about that. There’s two very different reasons for Seattle and the Bay Area.

The Bay Area/Silicon Valley is the most straightforward, to me. In the mid-20th century, the San Francisco Bay was a strategic military port. This caused a significant amount of government funding in industry and infrastructure, as well as research laboratories with interest to the MIC (the location allowed close collaboration between researchers, industry, and the military, with additional ease in keeping the strategic labs well-guarded). Research labs need researchers. So, naturally STEM-focused research universities in the area sa, an increase in funding to create a pipeline of scientists.

William Shockley, renowned racist, eugenecist, human embodiment of a gaping asshole, and one of three researchers at Bell Labs to invent the point-contact transistor became enraged when, instead of giving him sole credit as he demanded for both the point-contact transistor and the FET, began secretly competing with his colleagues to invent yet another type of transistor and give them no credit. Within about a year, they refused to continue working with him, on account of him being a gaping asshole. Five years later in 1951, his invention of the bipolar junction transistor was announced.

What does a gaping asshole who spite-invented a new type of transistor in New Jersey while making his colleagues hate him have to do with the establishment of Silicon Valley, all the way on the other side of the continent? Well, in 1953, he quit Bell Labs, likely after making more coworkers’ lives miserable and took a position at Caltech in Pasadena. The poor staff at Caltech were probably thinking that they had won big by getting THE inventor of the BJT, which would become the first truly widespread transistor in consumer products by the end of the 60s, they were, however, likely not fully-aware of how huge of a gaping asshole he was. In 1956, he quit Caltech and founded Shockley Semiconductor Laboratory in Moutainview, CA. He was such a huge gaping asshole that eight of his best researchers quit in 1957, directly because of him and went on to found the No Shockleys Club, officially known as Fairchild Semiconductor (this caused immense damage to Shockley’s company, which saw a takeover three years later).

Fairchild Semiconductor and companies started by those related to it continued to research and manufacture silicon semiconductors and integrated circuits. This was pretty much the origin of Silicon Valley (coined due to the silicon used by the semiconductor industry). The semiconductor firms formed a symbiotic relationship with the universities and research labs using them also as a pipeline for highly-educated technical staff and researchers.

How did it go from the semiconductor industry to software? Well, the same pipelines also produced people who knew how to progam the computers that these companies made with the advent of the microprocessor. Pretty simple.

Seattle. Well Seattle is weird. It became a tech hub because of the 1962 World’s Fair (the origin of the Space Needle). Seattle was a bit of a podunk town with Boeing also there (weird juxtapositon, having come from the area) and wanted to put itself on the map. They won the bid for the Fair in 1955 with the theme “Century 21”, all about an optimistic vision of the future. With the space race on, the Federal government was more than happy to dump bucket loads of money into the city, especially in STEM education. In 1962, both Bill Gates and Paul Allen attended the Fair. They founded Microsoft in 1975 in Albuquerque, NM, likely due to it being the home of Altair-8800 manufacturer MITS, who were their first client. By 1979, they had little success in finding local staff, so, they moved the company to the Seattle area, where they knew that STEM workers could be found.

tl;dr: Companies went where they could find talent. Also, William Shockley was a gaping asshole.