There is actually a somewhat decent answer to this question:

The USSR lasted a very long time, from 1922 to 1991. Unlike the Third reich. And if there was one thing the USSR was good at, it was censorship and control of informations.

When the nazi regime fell, the truth about the Holocaust and all the atrocities commited under the Third Reich quickly came to light. It allowed documentaries on the matter to be created relatively quickly.

But, while atrocities in the USSR were an open secrets for most of it's lifespan, it's only when it fell, when the archives were finally opened that peoples realized the true span of it's failings and political terror. Only after 1991 did peoples really started to understand why the USSR had been a bad regime, when peoples could testify without fear of being a target and archives were open for all to see.

In short, you see more documentaries about the Shoa because there has been more time to make them.

Herm, I wouldn't say the Holodomor of Ukraine was exactly an 'accident'.

It's pretty well know by now that Staline saw Ukraine as a dangerous place of potential political dissidents and deliberately enginered the famine to kill any attempt at an uprising before it even begun.

While plenty of food shortage in the Soviet Union were simply the fruit of incompetence rather than a an attempt at a genocide, the Holodomor definitely was. It was a man-made famine.