War... War never changes... The Fallout games have been around since 1997, and even though the series took a decade break until 2008 (when Fallout 3 was released), the hype around the universe never went down.
Fallout is set in an alternative history timeline after World War 2, where the semiconductor was never invented. Sure, there were transistors around (Mister Handy, slave collars, transistor radios), however they never became popular in the Fallout universe. Vacuum tubes continued to be used in electronics in the 21st century, and chips (eg. the "water chip") were still using them. Besides, because of the ongoing war, vacuum tube electronics would be less susceptible to EMP, which makes perfect sense, when you think about it.
This was done deliberately because Tim Cain, producer and lead programmer, wanted to "to explore more of the world and more of the ethics of a post-nuclear world, not to make a better plasma gun." So, while Fallout takes place into the future, it is not our future, but a throw back at the 1950s style science fiction "World of Tomorrow," filled with our hopes and dreams.
In 1997, players took control of the Vault Dweller from Vault 13, who had to leave the protection of their home to find a replacement water chip. What ensued was a grand adventure across the Wasteland, filled with interesting characters and fantastic RPG elements, that every Fallout fan should be acquainted with by now.