Lenin was never a military leader so never had a uniform.
For the first half of his rule, Stalin was at war with Germany and the uniform was a sign of their joint struggle against the invaders etc. After the war it was a good reminder of the struggle they had faced together.
Krushchev was never really a soldier and didn't wear a uniform.
I don't remember Breznev particularly wearing a uniform - except perhaps at military parades. He was a major-general in the war but only really a political office rather than a frontline commander - so the army may have taken a dim view of him pretending to be a real soldier.
Weren't Lenin and the other Soviet leaders dictators?
Dictators is a bit of an immotive term. By one definition Hitler was a democratically elected politician, while the Dalai Llama is an absolute religous dictator
A uniform is a costume like any other. Oil state Arab leaders dress in traditional desert robes to re-assure their subjects that while they might be jetting off to Paris really they are a historical tie to the desert.
Typical south american right-wing military leader dress up as cartoon generals to make it clear to the army that they consider the army of extreme importance, and to make it clear to the people that a proper army is in control. Typical south american communist leaders dress in simple green fatigues of the regular soldier to show the army that they are one of them and the show the people that they are humble leaders of the peasants.