From an earlier answer of mine

The Nikolai stems from a number of Lenin's writings produced during the First World War being signed "N. Lenin." A number of non-Russian contemporaries like Louise Bryant filled in the N for Nikolai, when it really meant nothing. The "N." was just a part of a larger pattern of Lenin using pseudonyms for articles he authored.

Lenin was not unique in this authorial attribution. The long and the short of it is that the adaptation of pseudonyms was quite common within the subculture of Russian revolutionary circles. There was a real practical reason for doing so. A pseudonym was a way to evade police surveillance. The tsarist secret police was quite active in monitoring, spreading disinformation, and occasionally flipping suspected revolutionaries. A pseudonym gave the police little to go on if they intercepted correspondence between revolutionaries. These small circles often knew whose face was behind the pseudonym, but the police would have to start from square one when trying to piece together these networks and their families. Moreover, the very size of Russia also made it hard for the tsarist government to monitor the whole of the country. Revolutionaries exiled to Siberia or kicked out of the country could evade recapture partly by adapting a fake name. So it made a good deal of sense for revolutionaries to use a pseudonym within this context. Lenin was not the only pseudonym Ulyanov used in his early career; one count has him using around 150 fake names.

There was another tactical reason for using pseudonyms. Russian revolutionary circles were often very literary in nature. Revolutionary groups printed a bewildering variety of books, newspapers, broadsheets, and pamphlets both within Russia and abroad. Lenin became the public pseudonym of Ulyanov in his broadsides in the newspaper Iskra (Spark). Most of this printed material was illegal under Russian law, so it made little sense to use real names and give the police an advantage. But there was another reason at work. Revolutionary circles were often quite small in number and pseudonyms were a way to give an artificial strength of the movement by inflating their numbers. Lenin was only one of the names Ulyanov wrote under for Iskra, although Lenin was his favorite and the one that carried the most global recognition.

Lenin did sort of reclaim his family name once in power. He tended to adapt V. I. Lenin once he became a major public figure and there was some occasional personal correspondence with signed as Ulyanov. The signature he developed was usually "V. I. Ulyanov (Lenin)" although there are some signatures he signed as just Lenin. His family background was not much of a secret within the USSR (although his Jewish grandfather was a skeleton in the closet) as was his elder brother's role in a plot against the tsar. His birth-town of Simbrisk was renamed Ulyanovsk after his death and the abortive Soviet nuclear aircraft carrier planned during the Gorbachev era was to also carry this name.

Much like the far stronger cult of personality that developed under Stalin, this maintenance of Lenin was partially cultivated by the state. As a good Marxist, Lenin would have eschewed a personalist cult of personality as typical of bourgeois dictatorships such as the Bonapartes' and during his lifetime, he tended to rebuke it. But a Lenin as an image was important for the securing and strengthening of the Soviet state and even Lenin reluctantly recognized this. But this was a cult of personality with a decidedly different orientation in its rationale and operations than typical personalist dictatorships. Stalin would reportedly once rebuke his son saying:

You're not Stalin and I'm not Stalin. Stalin is Soviet power. Stalin is what he is in the newspapers and portraits, not you, not even me.

This anecdote imparts an insight into how names and images mattered to Soviet leaders. It was important to the Bolshevik leadership to craft an intelligible and useful narrative of the past. That meant emphasizing "Lenin" as a symbol of consistent party organization prior to 1917 and how this pseudonym stood in for the greater revolutionary whole.