New Another factor is that a lot of visitors don't find many of the most venomous species to be super-impressive as exhibit animals.
I've noted this before too, venomous snakes are often very underwhelming to the public. They're typically lazing around doing nothing, and not nearly as big as most people think. They don't live up to the fierce reputation the media has given them.
That depends A LOT on the species, the specimen and the husbandry/representation in question. There are various species, in particular of the Trimerusurus genus that are so stunningly beautiful that it makes non-venomous snake keepers green of envy (slight pun intended). The white Southwestern Speckled rattlesnake from Yuma county, the blue Lesser Sunda Island pit viper, the red European Sand vipers from Montenegro, the golden Sumatran spitting cobra, the b & w Siamese spitting cobra, the horned Saharan Sand vipers etc. ...at WdG impress even ophiophobic visitors, who were dragged in by their kids / mates, while the curious behavior of our snouted cobras or inland taipans and the rare rattling of one of our rattlesnakes fascinate young and old. A massive Black mamba (or Forest cobra, West Mexican rattlesnake, Mangshan viper, bushmaster etc.) specimen might not be gaudily colored, but its mere presence does leave a lasting impression when presented accordingly. Sure, snakes and reptiles in general are not as hyperactive as some mammals (especially of the human variety), but they have their strong fanbase among zoo visitors. If I had gotten a dime for every time a parent has said "My child LOVES snakes" in my now six years of running WdG, I could have bought myself at least two spider-tailed horned vipers by now...

And venomous snakes, in particular some famous or at least beautifully colored species, are, still today, considered among the distinctions of a truly professional reptile collection in a major zoo. Sure, you don't need to have them, but they do make a difference in the public perception. And yeah, there are reptile curators and zoo directors who exclude them - for rational, fully reasonable reasons. [And because they are more into chelonians...

] But once the passion (not "thrill") for them has gotten into you, it's hard not to include them.
Hyped up expectations due to exaggerating medial representation are a general issue with certain species, especially in regard to charismatic mammalian megafauna in zoos. Which is both a toehold for your educational narrative as well as a good counterargument whenever the anti-zoo lobby comes up with its "You can learn everything about an animal by watching movies" mantra.