At the beginning of natural selection many snakes will die due to predators or lack of pray.
As time went on the different types of snakes needed ways to kill their pray. The non-venomous snakes would squeeze their pray and the venomous snakes would bite their pray with the venom.
In the harsh environments of which these snakes live, there are other predators that they need to compete with..
The struggles that snakes need to deal with are to do with the heat from the sun, danger from predators, and scarcity of water.
The venomous snakes have a higher chance of surviving due to the fact that they do not have to wrap themselves around and coke their victim.
After many years of natural selection there will be more venomous than non-venomous snakes.