Some possibilities include:
Emotional Attachment: Your ex might still have strong emotional feelings for you and finds it difficult to let go completely.
Insecurity: Starting a new relationship doesn't necessarily mean they have moved on. They might be seeking validation or using you as a backup plan in case things don't work out with the new partner.
Regret: Your ex might regret the breakup and want to rekindle the relationship with you, even if they are seeing someone else.
Control: Some people try to maintain contact to exert control or manipulate the other person's emotions.
Loneliness: They could be using you as a source of comfort or support, especially if they feel lonely in their new relationship.
Familiarity: Being with someone familiar might offer a sense of comfort and security, even if they are with a new partner.
Closure: They might be seeking closure or trying to resolve unresolved issues from the past.
Regardless of the reasons, it's essential to establish clear boundaries and communicate your feelings honestly. If you don't want to maintain contact, be firm in your decision and limit or cut off communication if necessary. Seek support from friends, family, or even professional counseling if the situation becomes overwhelming or harmful to your well-being.
Remember, your emotional well-being and happiness should be a priority, and you have the right to protect yourself from unwanted attention or interactions.