I remember answering a somewhat related question, at least in the part concerning the Czechs, in this thread. An important point to make is that asking why something did not happen – that is, the Germanization of Bohemia – is always rather tricky, and it is tempting to counter with another question. In this case, why should it happen at all in the first place? By the time the Polabian tribes had been the target of German military expansion from about the 10th century onwards, Bohemia was already an established polity for about two centuries, with a shared religion and close ties to the German people and the Holy Roman Empire and its rulers. In fact, Bohemian nobility invited and welcomed German settlers coming into the less developed border regions during the middle ages.

There were, however, significant and relatively successful efforts at Germanization of the country much later, under Habsburg rule. This is an extensive topic in itself and I can expand on it if you wish. In the meantime, following the 30 Years' War, Czech culture and language was being pushed to the countryside, with the cosmopolitan areas and upper classes under the influence of a sizeable German-speaking elite. This would in turn lead to the nationalistic backlash of the Czech National Revival in late 18th century, which culminated during the 1848 revolutions. Still, even after the dissolution of the Habsburg monarchy the presence of a German (or German-speaking) minority played a very important part in the country's history, being a key element in both the founding of Czechoslovakia in 1918 – with the semi-artificial Czechoslovak identity serving as a sort of a counterweight to the large minority population – as well as twenty years later when the presence of Germans in Czechoslovakia was used by Nazi Germany to achieve its goals during the Munich crisis.