The Soviet Union wasn’t overrun and occupied by invading armies.
Yes I think that is partly an explanation. The author touches on that when he talks about the peaceful end of the Soviet System: Gorbachev and Reagan had an understanding that no military advantage would be taken by the West, and Bush soft-pedaled the dissolution of the USSR with nauseating nonchalance.
Nevertheless nothing prevented the media from pressing for greater access to the archive and more historical research. They could be making triumphalist pieces in press and on television about the evils we've overcome. Instead, whatever research was done was buried by the media. For example, remember Venona intercepts? That could have been a matter of countless spy stories at the same time upsetting the conventional wisdom about McCarthyism. Nothing happened. That is because the identity of the media is rooted in the far Left political movement; they defend their own and the gathering and dissemination of information, that is, journalism is merely a cover.