In the Fallout Universe the transistor is not invented untill a decade before the great war. Technology in the Fallout Universe never entered the period of miniaturization that digital computers and electronics experienced. Instead they rely more on vacumm tubes. Creating a large divergence between the Fallout ATL and OTL.To never have computers invented at all would require some huge butterflies, IMO. Technological progress isn't necessarily a straight line, so it's not a given that computers "as we know them", so to speak, would always exist. But I do think it's a given that, as time goes on and technology evolves in any form, machines used for increasingly complex calculations will come into existence. That said, though, if no such machines do come into existence, the pace of technological progress will be much much slower. Not only due to those complex calculations, that'll take longer to solve, but the inability to communicate as quickly as we can with computers.
http://fallout.wikia.com/wiki/Divergence
http://fallout.wikia.com/wiki/Transistor
You might want to google Fallout electronics to see how bulky it is.