An important thing to remember is that the US and Western Allies not only thought Soviet agreement to participate in the Marshall Plan was unlikely, but they did not *want* such participation:
"Molotov and Novikov had a firm foundation for their suspicions. Most of the available evidence indicates that the Western powers designed the aid program to ensure that Moscow would not participate. The Americans, the British, and the French all agreed that Soviet participation would lead to protracted bargaining and delays in implementing any plan, since the suspicious Soviet leaders would be sure to impose many difficult conditions on their participation. The West Europeans and Americans were united on the need to move quickly. If, as Marshall put it, “the patient is sinking while the doctors deliberate,” then endless haggling with the Soviets had to be avoided.35
"In the days following Marshall’s speech, both the British and French governments scrambled to put together a rapid response. Desperate for the credits which Marshall was offering, they were nevertheless on the horns of a difficult political dilemma. For reasons of efficiency and strategy, noted above, they did not want to include the USSR in their plans, but, on the other hand, they had to put together an all-European plan of some sort for by the summer of 1947 it had become clear that the U.S. Congress would not approve any further piecemeal aid to individual European countries. American legislators felt that too much aid had already been sent into the “black hole” of the European economies. Thus any response to Marshall’s plan—if it was to pass muster in the American legislature—had to take the form of an all-European plan which held out the prospect of re-creating in the near future a self-sustaining European economy. The rub was that putting together such a plan also had to include a visible effort to include the Soviet Union, because in France and Italy, two countries whose participation was deemed essential to any successful program for Western European recovery, joining an economic plan which overtly barred Moscow would be politically unacceptable. In both countries, socialist-led governments had only just that past spring excluded the Communists from governing coalitions and were hanging on to bare majorities in their respective parliaments. Any further action which would antagonize the Left—as the deliberate exclusion of the USSR surely would—might throw these countries into a political crisis. As a result, even though British Foreign Minister Bevin and his
French counterpart Georges Bidault did not desire Soviet participation in the American aid program, they felt constrained to invite the USSR to collaborate in the initial planning.36
"When Bevin and Bidault met in Paris a few days after Marshall’s speech, they were careful to transmit to Moscow their desire to enter into consultations with the Soviets about a European response to the American initiative. As Bevin reported to London, “the main concern of the French government was to disarm domestic criticism to the effect that Russia had not been given in good faith a full and cordial opportunity to join in the discussions at the outset.”37 Thus, following their discussions, Bevin and Bidault extended an invitation to Molotov to join them at a meeting to be held in Paris the week of June 23. Both Bevin and Bidault assured the U.S. ambassador in Paris, Jefferson Caffery, however, that the invitation was little more than window dressing to defuse potential leftist opposition at home. Both separately informed Caffery that “they hope the Soviets will refuse to cooperate and that in any event they will be prepared to ‘go ahead full steam even if the Soviets refuse to do so..."
As for the Soviets, it is true that--given the vagueness of Marshall's initial speech--they did not at first want to reject the plan outright. But it seems very unlikely they would accept it in the form the plan was actually to take. This was in part because of fear of US penetration of the economies of eastern Europe but also specifically because of the German question: "In addition, the issue of Germany again proved divisive.Molotov wanted assurances that any German participation in the aid program would not jeopardize possible reparations payments or lead to an increase in German industrial capacity. The British and the French fudged the issue, but refused to agree to such terms. When it became clear that the French and British would not agree with the Soviet proposal, Molotov delivered a harsh denunciation of the Western states and stalked out of the conference. Essentially, the Western states had attempted to impose exactly those conditions which the Soviet leaders had defined as unacceptable in their pre-conference analyses..."