Punyaaskshi or Kanyakumari is an incarnation of Parvati. She took this incarnation to kill an asura, being Aadishakti and Jagdamba, she is the saviour and mother of the Universe.
The asura Bana through his severe penance got a boon from Bhrama ji that only a young girl or let’s say a virgin girl will be able to kill her.
So, if Shiva Shakti would have united again, then the whole purpose of this incarnation would have failed.
Mahadev can never deny someone if the person calls him with pure devtion,here, the person was Aadishakti herself, so he agreed for marriage, but for the greater good of universe and Dharma devas played their trick and didn’t let them marry resulting into destruction of devil Bana.
And Shiv Shakti united again.