The reason is simple: if she would find herself a Belgian prince, there may a problem with the law. Belgian legislation was introduced shortly after the Belgian independence in 1830 - in which Belgium had gained independence from the Netherlands - to state that a marriage between a Dutch and Belgian prince was forbidden.
This decision was driven by fears that Belgium would be reunited with the Netherlands in the case of a Belgo-Dutch marriage. However, considering the present time frame, we can imagine that a possible marriage would no longer be a problem. Photo below: the four children of the Belgian king and queen, with the princes Emmanuel and 16-year-old Gabriël (first and second from left respectively).