Why are some snakes poisonous and some are not?

The simple answer is evolution ! Over millions of years, different snakes developed alternative methods of subduing their prey. While some evolved strong muscular bodies for constricting their potential food, others modified their saliva into venom with varying toxicity.

Second answer, amplifying on the first

It's chiefly because different species of snakes are adapted (by evolution) to different methods of hunting and different prey. Snakes that take immobile or helpless prey such as eggs and nestlings don't need and haven't evolved venom, though they might have (for example) adaptations for climbing trees. And pythons, for example, capture pry by encoiling and kill it by preventing its breathing (constriction), so they don't need and haven't evolved venom either.