Question on the 4th interview

Any kind one can enlighten me? As someone born after Nixon's times, I have no idea on the actual atmosphere and the progressive intricate turn of events. When I watched the movie with the revelation on the 4th interview about the "previously unknown conversation between Nixon and Colson", I found it (as nice a plot turn as it is) hard to believe that a piece of information with such significance could have been ignored by both pro and anti-Nixon camps--surely both sides must have combed through all possible documents to turn the tide, no?

I conjure up possible explanations:
1. By the time this happened, Nixon got pardoned already and any incriminating evidence is deemed "not so important anymore"
2. Nixon was perceived to be guilty to such certainty that such incriminating evidence is not really treated as that important (but then Nixon would not have been taken by surprise about it, no?)
3. It takes someone with as much expertise as James Reston Jr. to be able to find the document whereas legal teams from both sides were not able to

In current times, it probably would have been hailed as a major break and would have been heralded somehow. But, it did not seem to stir too much responses...(at least in the movie) So, this bit really eluded my understanding...

scratching the head,
Richard