Interestingly what was unusual was that Michelangelo had a last name to give. Most people did not. You might know his predecessor, Leonardo da Vinci, but did you know that da Vinci is an indicator of birthplace, not a last name? Leonardo's full name was Leonardo di ser Piero da Vinci, or Leonardo, son of Piero, from Vinci. He had no last name! Thus, he was differentiated by his father and his birthplace, although later he made quite the name for himself. As for why Michelangelo being remember only by his first name, I would say that's not entirely true. Most art historians know his name to be Michelangelo Buonarotti, Michelangelo di Lodovico Buonarotti Simoni in full, and it might just be the general (modern) public picking up on Michelangelo's rather unique first name.