Venom production is a "hard" adaptation to achieve**. So it is conceivable that simpler organisms had enough evolutionary time to get the adequate mutations for this to happen, which would explain why venom is almost absent (I know only of a platypus as being poisonous) in mammals and birds, and seemingly abundant in fishes, amphibians (such dart frogs) or reptilians.
Also, as I understand there should be great selective pressure and mutation in simpler organisms is more likely to produce viable offspring than in more complex ones, which can be a contributing factor.
It has come to my attention that the answer I gave is not exactly correct. Upon investigating venomous mammals many more came up (not only platypus - male platypus to be more precise), such as vampire bats, solenodons, many types of shrews (thanks @releseabe), moles and even primates (the slow loris , being the possible single candidate)
There are no birds know to be venomous, but some (such as the pitohui, the ifrita and the roufous) can be toxic to eat.
My initial belief that toxicity is hard to achieve (this is hardly my area of expertise, and perhaps I should have refrained from answering something incomplete), does not seem to be really substantiated by literature, where lots of different organisms seem to have developed toxic/venomous adaptations.
Maybe it should be noted that many of these animals have evolved poisonous glands from salivary glands/sudoriferous glands and that they are described as presenting convergent evolution. Another caveat is also that most(all?) poisonous birds have Batrachotoxins that comes from their diet (which they concentrate on their skin and feathers), not something they produce themselves and are not as potent as the ones produced by dart-frogs. In this sense they are maybe less toxic than other animals, and the intensity of this effect may be related to co-evolution of those organisms and their prey/predators.
It's also worth mentioning that the slow loris, while considered venomous, has the poison from its gland that resembles a cat salivary allergen, and maybe would be better described as allergy inducing, rather than a poison, or perhaps cats could be considered venomous (my opinion, hardly worth anything). Another possibility is that the substances used in marking territory are the same and eliciting allergy is an unintended consequence (from the paper: "Rather than possessing a brachial gland like the loris, small cats directly add salivary allergen proteins to inanimate objects (... ) The idea that species recognition systems share a close relationship with immune recognition").
My opinions about toxic and venomous animals changed as I was reviewing this subject and, as I view it now, it is a property that is ubiquitous on many different branches of the tree of life .
Wikipedia link about venomous mammals. it is not very good to reference because people may change it at some point and the information to which I am refering would no longer be there, but it is strongly peer reviewed see here
shamelessly stolen from wikipedia about slow loris venom characterisation Talking Defensively: A Dual Usefor the Brachial Gland Exudate of Slowand Pygmy LorisesLee R. Hagey, Bryan G. Fry, and Helena Fitch-Snyder
pitohui: JP Dumbacher, BM Beehler, TF Spande, HM Garraffo, JW Daly. Homobatrachotoxin in the genus Pitohui: chemical defense in birds?
slow loris paper: Nekaris, K. Anne-Isola, Richard S. Moore, E. Johanna Rode, and Bryan G. Fry. 2013. “Mad, Bad and Dangerous to Know: The Biochemistry, Ecology and Evolution of Slow Loris Venom.” Journal of Venomous Animals and Toxins Including Tropical Diseases 19 (1): 21. https://doi.org/10.1186/1678-9199-19-21.