Getting back together is complicated when your ex is with someone new/rebound. Their needs for contact, conversation and connection are partially or fully being met in the new relationship, so they have no real urge to want to reach out.
Most exes are also careful not to send the “wrong message” by initiating contact. They are not sure if the new relationship will last but they also don’t want you to think they are leading you on.
Some exes do not want the new man or woman to find out that they are talking to an ex. It’s easier for them to explain responding than it is to explain initiating contact.