Expected 70-Year Persian "Sojourn"

The expectation going into the Persian Period was that it would be a repetition of the earlier Hyksos Period (and the even earlier "1st Intermediate Period" between the Old and Middle Kingdoms in Egypt). That is, the primary royal court would return to Egypt after the roughly 70-year interval.

The royal family apparently decided to break (or extend) that cycle by keeping Egypt in a more-or-less ruined state. Hence, Jews were not sent back to Egypt to rebuild but to Israel.