Basically bacteria, fungus and oxidation are what make food go bad. Without these things, decay doesn't happen. In fact before certain bacteria and fugus existed on earth, vegetation would just pile up in massive amounts. If I am not mistaken, this is why we have oil and coal. Massive piles of vegetation built up, which allowed for those to slowly convert to fossil fuels, since they didn't get eaten by bacteria and fungus.

So basically the canning process involves a few things.

• Cooking
• Preservatives
• Air tight seal.

You cook the food until all contaminants have died, then you seal while it us hot, which removes most of the oxygen. Since the hot air is thinner, when it cools it creates a vaccum seal with minimal oxygen, and prevents anything else from entering. Canned things can have vinegar, salt, citric acid, or other preservatives also, those also prevent any bacteria growth.

You now have a sterile environment that doesn't allow for decay, at least for a very long time. It isn't 100% perfect though.