Because France and England's guarantee towards Poland was 1) entirely a deterrent/bluff meant to Force Hitler into the negotiating table and 2) Was specifically structured in a way that would address Nazi Aggression.

For point 2, Britain and France guaranteed Poland's independence. I.e. the aim was to avoid a repeat of the Anschluss (Austria being absorbed into the German Reich) and Munich accords (Czechoslovakia losing its independence and becoming the "Reich Protectorate of Bohemia and Moravia). They wanted Poland to continue existing as an independent Nation. Of course, when Hitler invaded, the pattern was repeated and Western Poland/Warthegau was annexed by Germany, and what was left of Poland became the "General Government", i.e. a German Colony.

The Soviet invasion was different. In 1920s, during the Soviet Polish War, Poland seized territory from Ukraine and Belarus after repelling the Soviets. The Soviet Invasion was characterised as the Soviets taking back these areas. Simply put, they didn't destroy Polish Independence (they allowed the Nazis to do that) but were retaking their territory, and thus not threatening Polish Independence.

For point 1, it's important to note that despite ostensibly declaring War, France and Britain did nothing. Even after Munich, the British were hesitant to go to war, and many still believed that if the "wrongs" of Versailles were addressed - i.e. Hitler be allowed to take back German territory - all would be well. Britain and France were ultimately bluffing, and indeed, the most the British did after Hitler's invasion of Poland was to send its military to France in preparation for a WW1 style war of attrition. It would not be later until 1940 that Hitler would begin military hostilities by invading Belgium and then France.