why can't winston learn anything about the past?

Winston can't learn anything about the past because the true past has been eradicated. The Party maintains that the past was simply made up of a small ruling elite abusing a huge underclass. The Party holds that they started the glorious revolution to bring about equality and brotherly love under the auspices of Big Brother. People who know the truth have died or been killed by the Party. Eventually, given enough time, the Party's version of history actually becomes the only truth that people know. All history books have been destroyed replaced with Party dogma. Winston does find an old prole in a bar but can't get anything substantial out of him. The old man is a drunk and rather ignorant. Unfortunately Winston is only left with fragmented nightmares of his childhood to find answers about the past