Why was control over historical memory vital to the Communist Party maintaining power?

I would argue that the Communist Party (I assume you are
talking about the one in the Soviet Union) needed to control historical memory so as to
make sure that it looked good when compared to what had come before.  It also had to
look as if it had been completely competent and consistent over
time.

By controlling historical memory, the Party could,
for example, make it seem as if it had never made any mistakes.  For example, the Party
did a lot of changing history by changing the contents of old photographs.  When the
Party purged people, it then tried to make as if those people had never been important
in the Party.  By doing so, it managed to maintain the illusion that the Party could do
no wrong.  If it had acknowledged that its own high-up members had "gone bad" it would
have been weakening its own hold on power.

So the point is
that the Party had to maintain this image of omnipotence and competence.  By doing this,
it was able to keep people believing in it, which allowed the Party to stay in
power.