Zinc oxide eugenol (ZOE) with catalyst is the preferred root canal filling material for primary teeth because it provides excellent antimicrobial properties and good sealing ability, which help prevent reinfection. Additionally, ZOE sets to a firm mass that is biocompatible with the surrounding tissues and can be resorbed at a rate compatible with the natural exfoliation of primary teeth. This ensures that the material does not interfere with the eruption of permanent successors. Other materials like calcium hydroxide may resorb too quickly or lack adequate antibacterial effects, making ZOE superior in maintaining primary tooth integrity after pulpectomy.

Reference: Pediatric Dentistry: Infancy through Adolescence, 6th Edition.