Because that stratum of British society is very much a genteel Mafia. Earl Mountbatten - "Uncle Dickie" to the Royal Family - had the ear of the Queen and the Duke of Edinburgh. As such, he got a lot of 'respect'. One word from him, dropped in the right place, and Camillas' family would have faced social and probably financial ruin. Furthermore, Charles needed his mothers' permission to marry, and the late Queen would have taken Mountbattens' advice on the matter. For Camilla to have accepted Charles' proposal, only to have the Queen refuse permission for the marriage, would have been embarrassing to the point of scandal. To many people still recalled the whole Princess Margaret/Group Captain PeterTownshend farrago and how it had divided the country. There was no appetite for another affair like that, and Camilla and her family heeded the wanings.

It is a peculiarity of a rigid class system that those at the top usually have far less personal freedom than those at the bottom! Especially when it comes to choice of partner.