Mainly other dynamics.
Indeed mountains surrounding Bohemia helped*, but there were many other reasons - e.g. one was higher population density, another one was that widespread use of Czech language in writing started already in the 1300s (by contrast of Polish only in the 1400s, of German already in the 1200s).
*By contrast Poland's location in the middle of the North European Plain wasn't good for blocking any kinds of movements.
Also religious non-conformism of Czechs played a role - already in the 1400s Protestant movements were popular among the Czechs (just to mention the Hussites) and later, when rabidly Catholic Habsburgs ruled Bohemia, the main bulwark of Czech identity also continued to be Protestantism.
Such level of non-conformism wasn't present in lands to the north of Bohemia, where there were no such religious differences between Slavic and German speakers. That difference only started to the east of Poland's border, and only in the mid-to-late-1600s, when Poland became firmly Catholic.
During the Thirty Years' War Czech Protestants were suppressed by Catholic Habsburgs, many of them fled and found refuge in Poland:
Sudeten Germans were very strongly loyal to the Catholic Church. They did not flirt with Protestantism even nearly as much as Czechs.
