According to Herge himself, the two first (and worst) Tintin stories were commissioned by le Petit Vingtieme as political propaganda. Herge claimed he did the stories with little enthusiasm as he would much rather make comics about native Americans who he was deeply fascinated by.
However, this claim is somewhat challenged by the fact that Herge actually did a colourised remake in his distinctive ligne-claire style of the original black and white Tintin in the Congo. Although he dialed the racism down from 11 to 10 (like how Tintin was now teaching Congolese school children basic calculus instead of teaching them about "your fatherland, Belgium") it is still vile and disgusting. It's only value today is as historical documentation of the sick perverted way white Europeans perceived Africans back then.
Tintin in the Congo is so racist that it could not be published in the UK until the 1990's.
Herge never redid Tintin in the Land of the Soviets. Being an incoherent stream of 1920's anticommunist propaganda tropes set to crude drawings with nothing resembling a narrative structure, it was too bad to be salvaged and Herge would later express embarrassment over the poor quality of the drawings and plot and refused several offers to reprint the story. Herge was redrawing his previous work during and immediately after WWII and around that time the crude anti-communism of the story would not have been popular with readers and would have made him look even more like a nazi.