תהלים, פרק ק״ו, פסוק ל״ה

The failure of the Israelites to remove the local nations from the land set off a devastating social and spiritual chain reaction. Rather than remaining separate, the people began to deeply mix and blend with the surrounding populations [מצודת ציון]. The primary approach among commentators is that simply living side-by-side and sharing a physical space with these foreign nations is what ultimately brought about a highly destructive outcome.

Because they allowed the local inhabitants to remain, the Israelites naturally intermingled with them. This physical closeness inevitably led to the next stage of their decline, as they began to learn from their neighbors and adopt their evil actions and negative character traits [רד״ק, אלשיך]. This gradual process of assimilation and moral compromise tragically unfolded despite the fact that God had explicitly warned them in advance about this exact danger [אבן עזרא].