If so, why did Bnei Yisrael sin afterwards with the Golden Calf etc?

The Remak answers, that Bnei Yisrael can only reach their peak of spirituality in Eretz Yisrael, for most of the mitzvos can only be observed there and not in Chutz La'aretz. The Land itself purveys spirituality, whereas the rest of the world doesn't radiate any holiness. Thus even though Bnei Yisrael themselves had been cleansed from their impurities their work was incomplete, for they were living in the desert and not in Eretz Yisrael. It was because they were incomplete, that they were swayed into sinning.