The meaning of the word socialist changes by country and by era. Social democratic is especially crucial in Germany because of the national-socialist phenomenon. The word is a popular ideal which was abused by despots and which lost favor after the 80's, mostly for it'sits use by despots, it's vagueness and lack of concencusconsensus. https://en.wikipedia.org/wiki/List_of_former_communist_states_and_socialist_states
Socialism in the 20th century started out as an ideal term for debate of human rights of expression, minimum wage, trade unions, in the 1920s. The 1920's1920s' ideal is how I was introduced to the concept of socialism in philosophy and history, and same goes for Bernie Sanders too, probably.
Socialism changes a lot by decade and Bernie probably associates socialism with the ideals of his father's generation from the 30's, rather than the recent historical corruptions of socialism used by Venezuela, Germany, Burma, Romania, LybiaLibya and others.
Usage frequency of various political terms according to GooggleGoogle Ngram Search: