Why didn't Britain declare war on Germany in 1938 over Czechoslovakia?

Because Prime Minister Chamberlain didnt want to fight over Czechoslovakia.... The 1938 Munich agreement between Chamberlain of Great Britain and Hitler of Germany stipulated that the Sudeten Germans residing in Czechoslovakia on the border of Germany should be reunited with the Fatherland. But when it came to reunite these Germans, Hitler saw it fit to take over all of Czechoslovakia. Chamberlain and Great Britain aquiesced to Hitler's invasion. As the Allies had no treaty with Czechoslovakia as they had with Poland - Hitler's invasion of Poland in 1939 caused Britain to declare war on Germany - Chamberlain stayed in office until May 1940 when Churchill took over. By then Chamberlain was a sick man and he died not long after.

(Chamberlain actually resigned over a vote of confidence regarding the Norway situation)