Answer:
The germans and the scots-irish were usually very poor, some of them arrived as indentured servants and only gained their liberty after a few years of unpaid work. They found it hard to establish themselves in the cities and in the coastal areas, where the land was either occupied or more expensive.
In the case of the scots-irish, the vast majority settled in the south and the best land was occupied by rich plantation owners. These new immigrants did not have the economic means to start their own plantations and buy slaves.
They decied to settled in the "backcountry", a region that corresponds roughly with the Appalachian mountains. The land there was cheap, but the risks were higher.