Originally Posted by Iluvbertie
The only reason I can think of is without being Andrew's wife they won't be able to be buried together at the royal burial ground at Frogmore. If Sarah is his wife then she could be buried beside him there but otherwise separate burials or he has to be buried somewhere else. Only royals and their spouses can be buried in the Royal Burial Ground.
There is possibly another reason. With their ties to Epstein and Maxwell, they could be called on to testify in court. If they are married, they cannot be compelled to testify against each other.