There are a number of reasons, really:

• There was no joint strategy against Germany yet - remember that Poland was conquered within a month, not much time for mobilizing forces and working out an offensive plan.

• French military doctrine at the time was based on defense. They had dug in behind the Maginot Line and expected Germany to attack first. Britain had to mobilize an expeditionary corps which took quite a bit of time. Any kind of large-scale offensive on the Allies' part would have to be done through neutral Belgium - neither France nor Britain were willing to do that.

• The experience of WW1 was still very deeply engrained in both Britain and France. It wasn't until the Blitz and the attacks on Belgium and France which put both countries in existential danger that the war effort gained full support.

• Some politicians in both the UK and France still hoped for a diplomatic solution to the conflict. Even though appeasement had failed, there were hopes that Hitler could be forced to give up Poland or reinstitute the Czech Republic.

• The German defenses towards France (the "Westwall") were overestimated by the French high-command. They did a probing attack, the Saar-offensive, but withdrew again after a few days.

All these factors put together led to what became known as the "Drole de guerre" - strange war in French, "Sitzkrieg" - sitting war in German or "phoney war" in English - soldiers on both sides digging in, sometimes in very close vicinity to each other, but not exchanging fire until the beginning of the German campaign against France.

Source: Raymond Cartier: Der zweite Weltkrieg 1939-1941, Lingen, 1985.