Because diplomacy is rarely set in stone but more importantly because while the Polish-British Common Defence Pact does specify that Britain would come to the aid of Poland were it to be attacked by a European power there was another less public protocol that specified in no uncertain terms that the "European Power" in question refered to Germany and Germany alone. While the Polish ambassador to the UK did bring up the topic he was told very explicitly that Britain would not be declaring war on the Soviet Union. They did issue an official protest of course but in light of the world situation at the time it was not exactly hugely impactful.

While France had an alliance with Poland too the support for that alliance was not particularly strong in military circles in the late 30's, there was also a strong anti-war sentiment, particularly among the socialists (which were very very strong in France at the time). Slogans such as "Mourir pour Dantzig?" (Why die for Danzig?) were heard nationally. There was also a strong sense that coming to the defense of Poland was pointless because Poland would not be able to fight for any amount of time, this sentiment was echoed both in Socialist and (usually) right-wing military circles. Since it soon became painfully clear that Poland would indeed not be able to fight very long the dialogue soon turned from whether or not to aid Poland to how a long term victory against the Nazis could be achieved and in this, even in -39, both France and Britain saw the Soviets as a valuable ally. Either as a natural barrier to German expansion in the east and trade partner or a powerful participant in a war against the Nazi regime.

The book The Baltic and the Outbreak of the Second World War by John Hiden and Thomas Lane has great material on the behind the scenes diplomacy in -39.