Several factors, really. Alcohol is made when yeasts eat sugars and crap out alcohol and fart CO2. Not all sources of sugar are the same, and not all sugar is fermentable. Not all yeasts are the same, either. Just like humans have selectively bred crops and animals to serve specific functions, we've (mostly unknowingly until recently with advances in microbiology) selectively bred yeasts. Some have higher alcohol tolerances than others, for they'll usually keep reproducing during fermentation until they make their environment so toxic from alcohol it kills them - and then we drink it.

Aside from that, things like water chemistry (dissolved minerals, acidity, etc.) Factor into healthy yeast reproduction (hence alcohol production). Water is the main ingredient in all alcoholic drinks, and it's ratio to fermentables usually correlates to alcohol percentage. Grape juice is packed with natural sugars. Cider and grain? Not so much. But there's different kinds of grapes, apples, and grains that all affect that fermentable sugar content. There's a whole lot of grain types that are used more for how they affect flavor than alcohol content: crystal malt, roasted barley, etc have been kilned which caramelizes some to most of the sugar in the grain. This renders the sugar unfermentable but gives the beverage certain flavors. Wheat, Rye, and Oats also don't add much sugar, but they do affect mouthfeel.