Actually there are venomous mammals however they're obviously not widespread. I believe venom could evolve in more mammals but hasn't because of lack of requirement for it.
Consider the primary species which are poisonous or venomous:
- Fungi/Plants (such as nettles)
- Some fish
Venom and poisons fall into two categories, as a weapon when hunting and as a defense mechanism.
My suggestion would be that what groups all these species is their relatively slow speeds. Many of them are cold blooded and are vulnerable in cooler environments. The poisonous fungi certainly aren't renowned for going anywhere quickly! Even some of the fish (jellyfish for example sting their prey to prevent it escaping while it's digested).
I believe that most poisons and venoms have developed to make up for the creatures' weaknesses.
By contrast mammals are mostly fast and active. They don't suffer in cooler temperatures and generally rely on their speed and senses to escape harm/catch prey (something a nettle rarely does). As such they've never developed a need for venoms.
As to whether more mammals could evolve venomous characteristics I don't see why not. After all it's a competitive world. If a rabbit suddenly developed a venomous bite it would certainly dissuade a cat from hunting it!
I leave you with this thought from XKCD: Snake Venom evolved from saliva, which means that it all started with a snake who's mouth was slightly more gross than usual...