The situation, as they knew it, didn't warrant emergency measures like Stalin's personal speech/appeal. He was too important asset to be wasted like that, It was a war but not a calamity - yet. The unprecedented extent of the defeat wasn't known so Molotov was sufficient for that task. Especially that he spoke for not for himself but for the Soviet Government and Stalin:
Citizens of the Soviet Union! The Soviet Government and its head, Comrade Stalin, have instructed me to make the following announcement
- one might say, indirectly it was a Stalin's announcement anyway.
In contrast to Hitler's naked dictatorship, the USSR was a collectivist dictatorship, even the demi-god Stalin didn't ruled directly but through the Government, Politburo, committees, Congresses, troikas. To all appearances, the decisions were made collectively, and Stalin was serious about appearances. It was his decisions and his successes, but mistakes were collective. All those contemporary
if HE only knew writings show this. Always the others were guilty he was at best kept in the dark about their transgressions.
And this was the second reason, as written by Dimitrov and I think in
Molotov Remembers - among the uncertainties of the first days he didn't want to commit himself, his authority to some mistake, false information, some irresponsible nonsense - for this Molotov was perfect.
So the silence was nothing but business as usual, nothing out of ordinary there. It was nothing unusual that Molotov spoke for the Government and for Stalin.
Stalin's speeches were good and powerful, he spoke in intimate terms - but the impression would wear thin, loose its attractiveness if overused.
Compare Molotov's bureaucratic:
The Government appeals to you, citizens of the Soviet Union, to rally your ranks ever closer around our glorious Bolshevik Party and our Soviet Government.
Our cause is just. The enemy will be defeated. Victory will be ours.
to Stalin's:
Comrades! Citizens! Brothers and sisters! Men of our army and navy! I am addressing you, my friends!
A grave danger hangs over our country.
The enemy is cruel and implacable. He is out to seize our lands, watered with our sweat, to seize our grain and oil secured by our labor. He is out to restore the rule of landlords, to restore Tsarism, to destroy national culture and the national state existence of the Russians, Ukrainians, Byelo-Russians, Lithuanians, Letts, Esthonians, Uzbeks, Tatars, Moldavians, Georgians, Armenians, Azerbaidzhanians and the other free people of the Soviet Union, to Germanize them, to convert them into the slaves of German princes and barons.
Thus the issue is one of life or death for the Soviet State, for the peoples of the USSR; the issue is whether the peoples of the Soviet Union shall remain free or fall into slavery.