Because the KGB had penetrated MI6, the British intelligence services and had double agents, Philby, Burgess, Blunt, Cairncross, and Maclean. They had been recruited while students at Cambridge University. Hence the attitude within the CIA that anyone coming out of Oxford was also potentially a security risk. I was pissed cause all those guys went to Cambridge. In 2010 I met a retired CIA guy, and he told me that it was more likely that the agency was just lazy about doing a more complex background check. But that was the reason that they gave at the time. I don't know if I still have the letter. I might.