I've heard many conflicting reasons. Things I've heard include:
-The speed at which Poland fell was too fast for any of its allies to react.
-France and Britain needed time to mobilize. (I find this to be problematic, since I doubt the war was a complete surprise.)
-They were uninterested in providing aid despite treaty obligations. (Although I think there was some reluctance to enter another war, I wouldn't be eager to blame them quite so much.)
Which of these reasons or others are true, and what had the greatest impact? Thanks for your responses.
Geography. The aggressor stood squarely between the putative guarantors and its victim. And it took time to send large British military formations even to France. Even then they proved numerically insufficient eight months later to defend even the country they'd been sent to.
The speed of the conquest was also a big factor. With the Soviet intervention military action became doubly unlikely as an effective defence of the Polish state. British leaders in particular had never been keen on defending Poland's 1921 eastern border anyway: September 17 made intervention not only physically difficult but politically problematical.
And then there's the wider political dimension. "Better Hitler than Blum" found its echo beyond France's elite. Leading British statesmen as late as May 1940 were willing to do a deal with Germany: why commit troops to head-on conflict when compromise might still be available?