the bombs dropped in like 2070 right so why did technology stop 120 years? besides the robots

Alternate universes or somesuch.

It's really not something that's supposed to stand up to much scrutinity. When you get down to it - it was an aesthetic that Interplay thought would make for a cool-looking setting. They added some details after the fact for "suspension of disbelief," but it's not like they started out with a scientific breakdown of a realistic world and arrived at Fallout; but rather the other way around.

It's "soft" Sci-Fi. The trappings are just there for show. It's like asking how a lightsaber works, or how the Millenium Falcon can break the speed of light without suffering from the ill effects of the laws of relativity.

Edit- also, whatever Ausir says; when he drops by and explains it all.

not again.... The general lines is that the 2070 is what "would be" in the future imaginated by the people of 1940/50... much like the famous german movie 'Metropolis'... or the jetsons on 60's

The computer technology in the world of Fallout is roughly 1970s, the society and culture is roughly 1950s. There are other technologies that are extremely advanced beyond modern tech like power armor and energy weapons.

Fallout is set in an alternate timeline that diverged from ours in the 1950s. In the 21st century, it looked like the future as imagined in the 1950s pulp science fiction. E.g. instead of transistors you have huge vacuum tube-based computers with artificial intelligence and monochromatic terminals, robotic servants, a pseudo-utopian society based on 1950s values, black and white TV, music from the 1950s considered all-time classics, and most modern musical genres never appearing, etc,. Weapon development was also different from ours and thus plasma and laser weaponry was introduced, 10mm became the most common ammo type, etc. The modern high-tech weapons were never created - instead, weapons based on how people in the 1950s thought 21st century weaponry would look like are used.

http://fallout.wikia.com/wiki/Divergence
http://fallout.wikia.com/wiki/Fallout_world

besides the robot weapons cars and so much more it did not stop it just kept a 50's theme

Well in the fallout universe I don't believe computer circuts (or something like that) were ever created and so creating complicated electronics was a hassle. I also think that there is alot of tech that has been utterly destroyed and alot that gets overlooked. Like the vaults are very incredible technological achievment considering they can survive at least 200 years, and maybe far longer with a proper way to avoid inbreeding. So they sort of lagged in technology a decade or so due to the strains of the continued cold war, and they stopped immigration so the culture remained the same until the bombs fell.