Expose the critters and the door is slammed. That's not a price many American journalists are willing to pay.
It's different in Britain. After the 2000 election, when Harris' lawyer refused to respond to our evidence, my BBC producer made sure I chased him down the hall waving the damning documents. That's one sure way to end "access."
Reporters in Britain must adhere to extraordinarily strict standards of accuracy because there is no Bill of Rights, no "freedom of the press" to provide cover against lawsuits. Further, the British government fines reporters who make false accusations and jails others who reveal "official secrets."
I've long argued that Britain needs a 1st Amendment right to press freedom. It could, of course, borrow ours. We don't use it.