Death & taxes
It's said that there are only two inevitables in life: death & taxes.
But travellers
don't pay taxes. They traverse the length and breadth of the land in a convoy of dilapidation, bringing nothing to where they briefly settle other than a transient increase in the crime statistics, and leaving nothing but filth and damaged property behind.
Perhaps their avoidance of taxation is just part of their strategy to avoid the otherwise inevitable grim reaper.
It might also explain the stench.