This area used to be know as the English Silicon Valley. I suppose in the beginning of IT, start-up companies could not afford offices in London, so used the cheaper (at that time) office space built on land that had become de-controlled after the construction of the M4.
I think part of the reason is the ease of access to both Central London and Heathrow, many of the big tech giants came over from the US and would have most likely wanted somewhere close to an air port.
Also I'm sure when the tech industry took off one of the big industrial areas in that part of the country had just been built.