Because that’s the best type of soil for them to grow in.
Optimal Cranberry Soil
The cranberry is Wisconsin’s official state fruit. They grow well there.
The reason that cranberries grow well in Wisconsin is because the land is very conducive to growing them in that region. The sand and peat bogs and marshes are the exact type of moist soil that cranberries need to thrive.
Cold in winter, wet, sandy and containing peat. The ideal soil for cranberries.
That’s why they are grown in bogs.
Hope this helps.