You'll have to explain why it's such a great travesty because it's not obvious.

Because the types of people that would believe in Holocaust-denial in 2020, are the same types of people that are fed by conspiracy theories when things are banned.

So what's the problem?

The existance of such ban. It feeds the conspiracy theorists, and it's just weird - why only this genocide, and not the communist one, which is increasingly denied thanks to Russian online presence? Why saying "Heil Hitler" and the gesture is criminal offense, but "Cest praci, soudruhu" (a Czech communist greeting) is not, and why the Hakenkreuz is forbidden, but the star, hammer and sickle is not? Why do statues of Stalin and Lenin and other communist leaders remain standing around the world? Surely you can imagine the outrage if it was Hitler's statue.

The laws shouldn't have such weirds corner cases of completely banning references to one thing and ignoring another, arguably worse thing.

Why is banning speech a great travesty?

Because it hides the truth. Perhaps someone will discover people you wouldn't expect involved doing things we wouldn't expect which may cause some aspects from being denied to cease to be true. Silencing one groups end up silencing another.

Why don't we have a law banning denial of other genocides like the Armenian genocide. There should be an equal concern for all genocides being denied.