As for reasons why people use rear panniers only:

Depending on the person, it could be out of cheapness or economizing, imitation (as you suggested), ignorance, experience and knowledge, simplicity, not caring, absence of research, lots of research, what they have on hand, what the bike came with, an uncompleted bike project, a transition stage, an experiment, 'good enough' thinking, absence of perfectionism, presence of perfectionism, contrarian tendencies, and probably other reasons as well. I've done it at different times for a variety of different reasons myself. Including having a variety of bikes set up differently, and liking to try different things and mix it up. And test what I've heard.