Leninists were, at a time, what trotskyists called themselves in opposition to "Stalinists", where as Stalin outlined the philosophy of "Marxism-Leninism" so it's defined in that old polemic I believe. Don't hear Leninism too often anymore though. I think Marxism is emphasized mostly because of how he able to outline dialectical materialism as opposed to his other socialist/communist contemporaries, and even if we say Marx was more incorrect than those who came after him he's still foundational or at least inspirational to a lot of the work that came after him. Mao built on Lenin's theories in incredibly important ways as well but calling myself a Maoist has different connotations like Leninist might have, and ppl generally accept MZT as part of ML so it's been an effective way of communicating the contemporary communist position (siding with Stalin over Trotsky, Sino over Soviet, MZT over Maoism).

Considering the many great communists that have existed, even those who never took inspiration from Marx, or who saw his work as accurate and useful but not central to their communist thought (Frantz Fanon comes to mind, never feeling a need to call himself a Marxist) I generally prefer a simple "communist" as it gives a bit less eurocentrism and doesn't pay special attention to anyone in a movement that belongs to the masses. However, "Marxism" as a name for the eternal science of dialectical materialism and it's application is very effective as a means of communicating this school of thought and valuable contribution he made in outlining it, where as terms like "Leninism" "Maoist" "Hoxhaist" "Marxism-Leninism" "Dengist" often refer to a specific polemic outlined by an influencial figure in the overall movement as opposed to the quality of the named people's contributions to this science or its application.