There was also the fact that silicon was already heavily entrenched in the industry by the time people started to tinker with the possibilities of GaAs seriously. At least that's what I understood to be the main factor after listening to Seymour Cray's talk which was posted here some time ago: https://www.youtube.com/watch?v=xW7j2ipE2Ck He talks about how difficult it was to convince large suppliers to work with GaAs at about 30 minutes into the talk.