Because the people in charge had the equivalent of many degrees in socialism, communism, and Marxism – they had written multiple books on the subject, participated in numerous conferences and lectures, and had spent most of their adult lives practicing and honing these philosophies. Lenin and Stalin believed wholeheartedly they were implementing communism throughout their entire lives and stated so multiple times. So why not call it communism?
Because there was a state, there remained a class system, and money was still in use. The three main things communism aims to do: stateless, classless, moneyless. The USSR was a socialist state, with an aim toward communism, which are the tenets of Marxism-Leninism (my emphasis), a form of socialism. Communism is a different form of socialism.
Lenin himself made this distinction.
