Malbim: The harvests lasted three months (Rus Rabah 5:11), the time for Havchanah. A convert may not marry before this [lest she give birth nine months after conversion, and we will not know if the baby was conceived before conversion or after marriage]. Therefore, after this time she strove to do Yibum. Rus clung to Bo'az' girls only as long as she needed to gather. After this, her love for Na'ami overpowered her love of all [the girls].