I suspect you're getting this data from the Wikipedia page on the deadliest conflicts in human history. If that's the case, you need to consider the casualties per year of a conflict. The First World War 4.5 years long. The mongol conquests went on for a period of over a century, and the Qing conquests lasted for 40+ years. The scale of the carnage in the First World War was highly localized in both time and space.

You also need to consider the social impact of the war. The First World War occurred in a time of modern communications. Civilians back home heard the news of battles and losses soon after they occurred. Armies were also citizens' armies in that most families had a family member or someone close to them fighting and dying in the war.

The war was also one of the first large-scale industrial wars, and the scope of the killing was different than most conflicts other than maybe the US civil war. Artillery was the biggest killer, and men under artillery fire could rarely see their enemy. Add gas, machine guns, and tanks into the mix and you have a totally unprecedented kind of mechanized and industrial warfare.

Lastly, we have far more records from the First World War and we are still living in its shadow. It's difficult to say the same for the other wars you've mentioned. We have soldiers' diaries that gave us a glimpse into what people at the front were experiencing. We have film. We have photographs. We know people who had great grandparents who fought in the war. In Canada and the U.K., the armistice day is a holiday.

In short, casualties aren't what make the war so impactful. They are part of it, but the war was a major watershed in many social, political, and cultural ways. We are also closer to it in time and collective memory.

I recommend reading some work from Jeremy Black. There is also a good book edited by Holger Herwig and Chris Archer on the history of warfare that should help you better contextualize the conflicts in question.