Short answer: because they didn't have to.

Longer answer: here is, for example, the article 1 of the Anglo-Polish Agreement of Mutual Assistance of 25.08.1939:

"Should one of the Contracting Parties become engaged in hostilities with a European Power in consequence of aggression by the latter against that Contracting Party, the other Contracting Party will at once give the Contracting Party engaged in hostilities all the support and assistance in its power."

Any European power, seems clear-cut, no? Well, not so fast. Here is the beginning of the secret protocol of the same date, attached to the agreement:

"The Government of the United Kingdom and Northern Ireland and the Polish Government are agreed upon the following interpretation of the Agreement of Mutual Assistance signed this day as alone authentic and binding.

1. (a) By the expression "a European Power" employed in the Agreement is to be understood Germany. (b) In the event of action within the meaning of Article 1 or 2 of the Agreement by a European Power other than Germany, the Contracting Parties will consult together on the measures to be taken in common."

So, Britain was not obligated to attack the USSR. Same with France, whose treaty with Poland was ratified only on September 4, 1939 (!) and was explicitly aimed against Germany.

The Soviets didn't know about the secret protocol, but their excuses (Poland no longer existed as a state, they were neutral towards it) were likely aimed at defusing the public part of the Anglo-Polish treaty.

The Polish ambassador Raczynski did in fact protest the Soviet invasion to the British and pointed to the 1st article of the treaty, Halifax responded that this only applied to Germany, which fact Raczynski conceded.

So, neither France, nor Britain were obligated to counterattack the USSR, and both countries thought it would be a mistake (plus they hoped for frictions between USSR and Germany, which now had no buffer zone between them, and thought the German-Soviet alliance would not last, at which point they could "jump in"). Which is why they did not declare war on the USSR.

Source: Keith Sword, "British Reactions to the Soviet Occupation of Eastern Poland in September 1939", The Slavonic and East European Review, Vol. 69, No. 1 (Jan., 1991), pp. 81-101.

Because the assistance agreement between Britain, France and Poland was after Germany invaded Chezchoslovakia and was really only aimed at Germany. Also, Britain and France couldn't do anything about the Soviet Union (or Germany in actuality but that's a separate point).

First, the Soviets dressed up their action as a counter to German aggression. There were rumors about secret cooperation between the two, but the details were totally unknown.

Second is... Blind chance. The order of events saw Britain and France declare war over German actions in Czechoslovakia and Poland before they got mad enough at Stalin.

That is, the internal wars in the USSR come at a time when the world is still war weary and anti-military. The Germans invaded Poland in the fall of 1939, and threatens France and England directly (these two states also helped create Poland to counter the Bolsheviks and Germans after WWI). The Phony War begins, then the USSR annexes the Baltic states and invades Finland that winter. The next spring Germany invades France, but the Soviets are content trading with Germany and expanding into smaller neighbors.