I have to respectfully dispute that premise.
I became aware of those facts decades ago, but Hitler’s holocaust, while certainly abominable, was the only example of mass genocide referenced in school and elsewhere.
It’s my guess that the masses are, for the most part, oblivious to the level of genocide perpetrated under communist regimes.
One can only speculate as to why that’s the case…
Another problem is the hypersensitivity to “racism”, as manifested by the concern over “genocide”.
IOW, apparently it’s OK to kill your own. Liberals only get upset when “others” are killed via “racism”.
I ask the question for those obsessed with that, does it make you feel better that all are equally killed?
Not much "speculation" is needed: we've been in the midst of a full-bore Communist takeover for the past 50 years at least. But it's been incremental and the frogs (us) are just beginning to realize we're being boiled alive.
Get a copy of "The Black Book of Communism" before it too is banned, to find out the true extent of Communist genocide. If everyday Americans realized the existential threat they're facing there'd be an instant armed uprising.