I have a more in depth answer I wrote on Tuva here. It had Russian settlement in the region (mostly for mining) dating back to the 19th century, and the reasons for annexation in 1944 were both strategic (to put more Soviet territory in front of the industrial Kuzbass region) and domestic (to discourage potential separatist ideas among other nationalities like the Oirats and Buryats by eliminating a nominally independent state of a "brother" nation.

Mongolia (technically Outer Mongolia) had different strategic uses for the USSR. Its independence was ensured by a relatively large Soviet military presence. I have some further info here, which also links to some info from u/kaisermatias. To restate it a bit: from the 1920s both the Chinese Nationalists and Chinese Communists considered Mongolia Chinese territory. Directly annexing it would have disturbed Soviet relations with both parties, and the Soviets already effectively controlled the nominally independent Mongolia anyway, so it would have been a big diplomatic loss for nor real gain. The border between Outer and Inner Mongolia wasn't even demarcated until the 1960s, so a direct Soviet annexation would have meant that the Soviets would have vastly multiplied their already-existing territorial disputes with China and would have had to commit major resources to defending Soviet sovereign claims. Which again, was not something they needed to do since they already had effective political, economic and military control over Mongolia anyway.