Exactly this, Slovenes as another example were under German rule for even longer than the Czechs, and kept their identity and language wholly intact. A barrier I see to Germanization of Czechs is a physical one: Bohemia is a land surrounded by mountains and very hilly besides. Historically there were Slavs residing in what is now Germany from the Elbe river and then East to the Rus, but these Slavs were Germanized rather quickly shortly after Charlemagne. Germans came in with Christianity and generally better military tactics, became lords (as bishops, counts, barons) over large swathes of forest and plain, and the Slavs living there assimilated.
Bohemia, on the other hand, as I said is full of mountains and forested hills, hostile to any enemy army or occupation. In addition, that very geography lends the region to internal unity unlike the loose villages of the Elbe-eastward Slavs, which is why Bohemia was ruled by native Bohemians or Bohemianized foreigners until the 15th century, which is when Germanization was attempted and, as mentioned, the Hussite movement extinguished any chance of the Czechs forgetting their identity.