13 But the Israelites did not drive out the people of Geshur and Maakah, so they continue to live among the Israelites to this day.
Joshua 13:13
ESV - 13 Yet the people of Israel did not drive out the Geshurites or the Maacathites, but Geshur and Maacath dwell in the midst of Israel to this day.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Because it was not part of God's plan as outlined in the eschatology portions of scripture (i.e.- Daniel, Revelation, and elsewhere in God's Word). That said, please note, that God held accountable Israel's sin in regard to their failure to carry out the commandments of God on the subject (refer scripture).