It was a result of Twinkie research.

Damnit! Beat me to it!

"Canned foods are sterile, so they won’t host bacteria, but eventually the taste and texture of the items inside will deteriorate" Personally with some of the canned food I've seen not sure how that is possible... So unless the packaging was damaged it should still be edible but just stale and loss of flavour. Guess that's why the old world food didn't heal as much as modern stake, punga or sushi (Mirelurk)

I view it more on the lines of "why hasn't anyone else taken this in the past 200+/- years?"

One could argue that the food was sterilized by the radiation, prevent microbes from decomposing it. Which would also explain why the player gets rad poisoning from eating food he/she finds.

However, they are holes in that. Namely there are bacteria which resist radiation, and cockroaches should still be able to feast on it.

Fallout universe is an aternative univers where science is wierd or alien then our own world.

They've found canned meat that's over 100 years old. Researchers didn't actually eat it, but it wasn't contaminated, and theoretically, it's still edible. And to be fair, we don't know if the salisbury steak doesn't come in a large, tray-like can.

SCIENCE!

Canned foods in our own universe can last indefinitely. Notice that canned goods do not have expiration dates, but "best by" dates. So long as the can isn't defective/damaged/bulging, you can eat canned foods years and years and years after that best by date has elapsed.
There were canned goods dating back to the civil war were opened up a few years back.
The taste and texture may not have been what they once were, but they were not spoiled.
Foods are cooked inside of the can after sealing them.

Throw preservatives derived by SCIENCE! into the mix? May as well be raw honey.