* Venom is biologically expensive to make. The first thing I'd check for is whether more effective venom is harder to make, thus some animals would make the easier kinds.
* Individual variation among both predator and prey drives an unending evolutionary arms race. If all the snakes had the same or very similar venom, it would be much easier for prey to develop resistance to it.
* Venom is not one thing, but a swarm of exotic and intricate compounds. That means some snake's body has to figure out how to make this stuff. They won't all think of the same things. A snake with cheap but effective venom would probably outcompete others, but if prey is plentiful, maybe not. If strong venom is costlier than weak venom, likely both will persist due to counterbalanced pros and cons. Similarly, some things may be easier to 'discover' than others.
* Check the genetic diversity, including the epigenetics. It may be that these complex formulae are affected by flipper switches that effectively randomize the production mix.