Because pure communism breaks as soon as you have more than a few hundreds of people living together, in my opinion.
We are not ants, and we as a specie are doing things more for ourselves than for others.
A hypothetical society wanting to approach to the closest version of communism would need to be terribly authoritarian and selective, and would be very vulnerable to non workers pushing down the economy. To live in communism means to not let freedom to the workers. It is as unsustainable as fordism.
Looked up on an encyclopedia, i admit i have been actually confusing communism with communalism, or communism at its primal sense. I had the idea that communism is the abolishment of private property and the equal repartition of remuneration between people, wether they work hard or not. Kind of like the functionning of ants. Simply got this idea from high school honestly (heh).
Though, if communism is only about collectivised production and distribution, i can see why it would be interesting to successfully implement it.
Honestly, i haven’t got a good enough knowledge of political alignments yet to be able to answer your question correctly, thank you for making me understand that. Do you reccomand any reads/authors who approached this topic?