With them went erev rav, “a mixed multitude”, many of whom were members of other races who had also suffered at Egypt’s hands – and probably a number of the Egyptian native population who were sure they would enjoy a better life away from their homeland.
One wonders why Moses showed no apparent interest in incorporating the erev rav into the ranks of his own people. It would increase the Israelite population and be a generous gesture to the people concerned.
It does not answer the question to say that the Torah was not interested in the future of the erev rav and so they faded from the story.
The better answer is that both Moses and the Torah had no wish to be deflected from the task of Israelite nation-building.
God had promised to make them a distinct nation with their own traits and tasks. Integrating a motley throng of fellow-travellers who shared neither the family history nor the religious and cultural tenets of Israel would create countless problems for the future.