Probably because even today there's no academic consensus that there was a campaign of genocide in Ukraine. Or to be blunter, as definitions of genocide vary, it is entirely unproven that "Stalin decided to replace bullets by a much cheaper medium of death, mass starvation". No one seriously disagrees that there was a famine but its causes and definition are still hotly debated.

Later research (most obviously Davies and Wheatcroft's The Years of Hunger) has tended, in summary, to place the famine in the context of disastrous economic mismanagement, poor agricultural practices and adverse weather conditions. Now the Stalinist elite, as authors of those economic policies, almost certainly has responsibility for the resulting crisis (throughout the Soviet economy) and deaths but this is very different from a deliberately engineering 'hunger plan' to starve Ukrainians into submission.

Does this 'prove' that there was no genocide? No. Some historians reject the folding of the famine into the broader economic narrative and will disagree. (Although I believe that even Robert Conquest, who popularised the events in the West during the 1980s has backed away from claims of deliberate genocide.) Which is fine, that's history. But it makes little sense to damn people or governments for 'denying' genocide in the absence of this consensus.

That includes contemporary governments. It was hardly unknown that there was a famine (despite Soviet denials) but then famines were unfortunately not a novelty in recent Russian history. It wasn't however clear that this was genocide because, well, it's still not clear. Those who denied that there was a famine (after the fact) entirely, or still do today (!), are simply a case of blind ideological loyalty.