Because the USSR was not at war with Japan at the time. The USSR and the Japanese were officially neutral until August 9, 1945.

Stalin did sort of expect and hope to be asked to sign the declaration (because it would make the transition from neutrality to war a little more straightforward in his eyes) but the US wasn't interested, because at that point they hoped that the war might end before the Soviets entered into it (one less postwar thing to deal with).

As it was, after the bombing of Hiroshima, Stalin told his marshals to speed up their invasion plans, and set up a meeting to inform the Japanese that they were tearing up the neutrality pact and declaring war the next day (though "the next day" didn't specify a time zone, so the invasion ended up beginning only an hour later).

Lots of details on this and more in Tsuyoshi Hasegawa's Racing the Enemy, which is all about the US, Soviet, and Japanese relations in the final months of World War II.