Weren't Lenin and the other Soviet leaders dictators?
Whether or not they were dictators is a separate question and one that does not necessarily relate to what they should or would wear.
Why did Joseph Stalin, Nikita Khrushchev and Leonid Brezhnev wear military uniforms when Lenin and the later Soviet leaders did not?
All three had specific reasons for appearing in military uniforms if and when they did (keep in mind that they did not regularly or always wear them). Stalin utilized the victory in the Great Patriotic War to establish his demi-God status throughout the Soviet Union and portray his actions in the 1930s as necessary for the war's outcome. He used the war as a tool and himself, or rather the idea of 'Stalin', as the ultimate genius behind the victory over Nazi Germany.
Khrushchev allied himself with the likes of Zhukov when he was attempting to take over after Stalin. It was under Khrushchev that the first military memoirs about the war were published (Stalin refused to allow much to be written about the war for fear of anyone asking the wrong questions). Khrushchev was a commissar but never really held any position of importance.
Finally, Brezhnev jumped on the 'War Cult' bandwagon because he tried to use the war to excuse previous failures and attempted to utilize the deeds of that generation to inspire the current and future generation(s) by having veterans visit schools to talk with students, by making May 9 a national holiday (it took 20 years for that to happen), etc. He was also a commissar during the war, lower ranking than Khrushchev, and his claim to fame was 'Malaia Zemlia'. This was an operation in the latter part of the war where a diversionary operation was so successful that the main operation was called off and all the forces earmarked for the main offensive were instead switched over to Malaia Zemlia (it was an attempt to liberate Novorossiisk in 1943). Brezhnev never played an important role, but if you look at the literature from the time period, he was apparently pretty damn important. And in that picture, you can see that he was only one of 4 people in the entire Soviet Union to receive the coveted 'Hero of the Soviet Union' Gold Star three times (the other three were Zhukov and two of the highest scoring aces in the Soviet Air Force).