While more can always be said, I hope these previous posts of mine might offer some basic ideas:
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The points are:
- Northern Germany (Saxony) was generally a not so preferred target of the raid except for the political rivalry between the Franks and the Danes in the 9th century, since Saxony was not so rich - There had been few wealthy church institutions and towns had not been so developed at that time. It is true that lower Saxony has Harz mountain region that produced silver, but it is located far inland and the first discovery of silver dates only back to the late 10th century.
- On the other hand, Both Russia and British Isles were located on the trading hubs in early medieval Eurasia, and the control of the trading network by settling in and contacting with the local people there were much more lucrative.
- As I also mentioned before in: The Danes or Vikings from later Denmark would often raid the Frankish Empire and later what would become the Holy Roman Empire. How were they so successful and how did the Danes avoid being conquered by the big powers?, military presence of the Germans in Saxony were strong in the 10th century, and it was the Germans who sporadically took expeditions in now Denmark. In other words, Saxony was neither a so easy prey for raiders.
Possibly Recommended Introductory Readings: