Have you ever wondered why there are so many pathological wallaby skulls for sale? And why the pathology is almost always an infection of the jaw?
These infections, known as lumpy jaw, are a leading cause of death of macropods in zoos and in the pet trade. Common causes are stress (wrong living conditions, stress caused by other animals) or being fed the wrong food. Macropods have sensitive mouths, they should not eat straw or tough hay. The tough fibers can cause small wounds in the mouth which can get infected, and without the right treatment the animal will surely die.
While it does happen in the wild and in zoos, most of the wallaby skulls you'll see for sale come from the pet trade. Wallabies are popular exotic pets in north-west europe, they're cute, not very expensive, and people keep them in their garden. Especially the fancy colored ones have become somewhat of a status symbol. People sell them on online selling/trading platforms (like the american craigslist) and don't even bother to check if the buyers know how to take care of these wild animals. The buyers never have the wallabies neutered, and in time they'll sell the offspring of their wallabies to unprepared buyers, and the circle continues.
In conclusion: please don't keep an exotic animal as pet, and if you do: 1. Don't and 2. Do your fucking research