High-quality preschool programs are heralded as effective policy solutions to promote low-income children's development and life-long wellbeing. Yet evaluations of recent preschool programs produce puzzling findings, including negative impacts, and divergent, weaker results than demonstration programs implemented in the 1960s and 70s. We provide potential explanations for why modern preschool programs have become less effective, focusing on changes in instructional practices and counterfactual conditions. We also address popular theories that likely do not explain weakening program effectiveness, such as lower preschool quality and low-quality subsequent environments. The field must take seriously the smaller positive, null, and negative impacts from modern programs and strive to understand why effects differ and how to improve program effectiveness through rigorous, longitudinal research.