The author of the excellent Stalin bio Stalin: Volume I: Paradoxes of Power, 1878-1928, Stephen Kotkin, has an interesting speculation on why Lenin's body was preserved in this way after his death. In 1922, Howard Carter's discovery of Tutankhamen's tomb in Egypt was widely publicized around the world, including in Russia where articles on the find were regularly featured in Soviet newspapers. Kotkin speculates that after Lenin's death in 1924, his body was preserved and displayed to the public as a way of cashing in on "mummy mania".

Kotkin further mentions that as the Russians had no real (recent) tradition of mummification, the preservation of Lenin's body was entirely improvised at the time. So it's somewhat remarkable that there's anything left of him at all by this point.

A little bit of topic but North Vietnam did the same with Ho Chi Minh despite he did not want that ( iirc he wanted to be incinerated and his ashes split in the country ) The communist party, after his death, overrule his testament and keep his body for the people, Lenin way. They recognized it some years later in a meeting.

Source : extra to the testament of Ho Chi Minh, introduction based on statement of the CP