People who made laws in 1919 knew why certain laws were made. 60 years later almost all of them were dead and people in power never experienced the issues earlier laws successfully prevent, so they did not consider them important.
That's good in some cases - nobody today considers God's mandation of kings, because people who did believe it are dead for centuries.
In other cases, it is bad.
This is often referred to as "Chesterton's fence", the principle that reforms should not be made until the reasoning behind the existing state of affairs is understood.
> In the matter of reforming things, as distinct from deforming them, there is one plain and simple principle; a principle which will probably be called a paradox. There exists in such a case a certain institution or law; let us say, for the sake of simplicity, a fence or gate erected across a road. The more modern type of reformer goes gaily up to it and says, “I don’t see the use of this; let us clear it away.” To which the more intelligent type of reformer will do well to answer: “If you don’t see the use of it, I certainly won’t let you clear it away. Go away and think. Then, when you can come back and tell me that you do see the use of it, I may allow you to destroy it.
We're seeing this also in other areas of society - once people who have experienced a traumatic event die, they cannot tell the tale any more and revisionists have it easier and easier to deny, alter or outright falsify records. Holocaust denial and the current pandemic are the best examples of the modern time - there aren't many left who have experienced KZs first-hand, and there aren't many KZ or Arbeitslager outside of Germany where people can see physical evidence. For the 1918 flu pandemic, we only have records - and those that we have aren't the most exact since bookkeeping wasn't as commonplace as it is today, records were lost or damaged as a result of fires, wars or discarding, and methods such as DNA/RNA analysis didn't even exist back then.
For more recent events such as the Balkan wars, the situation is a bit more complex - we have lots of survivors, physical evidence (mass graves) and court documents, but nationalist politicians have managed to create massive intimidation campaigns against those who dare to speak up against "the truth" (as defined by the government). For China and its human rights violations (Tiananmen massacre, Tibetan and Uyghur genocide), economic and geopolitical reality prevents standing up.
The result is that people can claim and do all kinds of bullshit, and with no eye-witnesses able/willing to stand up against this and archives incomplete or difficult to access it is also harder for regular people to stand up and counter these narratives.