Um, they very much did make promises to that effect. Neither were in good position to actually help the Poles when push came to shove, hence the Phony War. Brittain did some good with their navy, but neither could get enough troops to where it mattered to help, so they buckled down on ramping up their own war efforts at home to better mobilize. Did they fo it out of cowardice and throw the Poles to the wolves, or out of necessity because they would have been overrun had they over commited? That's a question that has been the subject of much study. But they both very publicly and loudly commit to their defense, they simply failed to meaningfully uphold that commitment.