In Ezra 2:21 the listing of the returnees from Bavel to Eretz Yisrael switches from lists of families to peoples from specific areas in Eretz Yisrael (the commentary of Daat Mikra identifies nearly all of the locations). I would like to know the reason for this change in identifier.
Malbim on the verse writes:
"בני בית לחם, אלה שהיו כולם ממשפחה אחת אף שישבו בכמה ערים צרף מספרם ע"ש ראש המשפחה, ואם לא התקבצו ממשפחה צרפם ע"ש העיר שגרו שם תחלה."
Translation: "The sons of Beit Lechem, those who were all of one family even if they lived in several cities he added their numbers together under the name of the head of the family, and if they didn't gather from one specific family he counted them together under the name of the city they lived in originally [that is, prior to the exile]."
According to Prof. Yehoshua Grintz in "Mechkarim Bamikra" (Studies in the Bible), pg. 179, the cities of Yehudah were badly destroyed by the Babylonians while the cities of Binyamin were considerably less so, so the people of Binyamin were able to return to their own cities, while the people of Yehudah had to rebuild practically from scratch. Because they had no cities (yet), they identified per family and not per city.