It’s because he/she wants to be and not with that other person that he/she used to be with before meeting you.
You might not even see or know what it is that is so special about you that your partner finds attractive and interesting in you, but that doesn’t really matter.
You might think that you have to compete with your partner’s ex, but unless you have a reason to believe that it’s not really over between them, you really shouldn’t worry about any such competition.
Leave your partner’s past where it belongs – in the past.
Your partner’s love life before he met you is part of his/her life and of who he/she.
It’s very likely that his past romantic experience allows him to be a more competent partner now who is less likely to make the same mistakes he did in the past. You should not expect your jealousy to disappear after talking to a friend or an expert once or twice, or reading one article or one self-help book.