It's a wasteland because the attack on the Omnic factory Roadhog organized went wrong, and the entire factory went critical, the detonation wiped everything clean. They hate omnics even more now.

To add to what Candles said, the omnics attacked Australia, took over the area Junkrat and the other junkers lived and the after the crisis was mostly stopped by overwatch they made a deal giving the omnics that land, the Junkers obviously said screw that and attacked to reclaim their homes.

In reality Junkrat and Roadhog are the victims of the omnics attacks, government overreach, and nuclear misfortune when they attempted to reclaim what was theirs, it is no wonder they outlaws.