The main reason Britain did not declare war on the USSR was because while the Anglo-Polish defence pact officially protected Poland from attack by any "European country", secret parts of the treaty specified Germany.

When the USSR invaded and the Poles argued that Britain should declare war on Russia because of the treaty the British were able dodge the issue by pointing to the sections specifying Germany only.

In practise Britain and France could do little to help Poland directly and Russian invasion basically destroyed the best strategy the poles had of falling back to the Romanian border and waiting to be resupplied/reinforced through Romania.

With Poland a lost cause and people not entirely expecting the war to become the next world war it didn't make that much sense to escalate the war and hand Germany a major ally.

Is the treaty part of why Poland wasn't freed from Soviet control post war?