Fior the simple reason that communism can not have an economy, needed in any (even village-sized) communities. Never mind province-sized, or country-sized or union-sized…
Communism can only ever be applied (if at all) in a small community. These will also fail, because comunism is an -ism an Ideological, Stupid Move (if you will). Contrary to that, capitalism is not an -ism, it’s what people do, if you leave them alone.
Remember the failed hippy communes from the ’60s?
Back on topic: global communism means nowhere no market, anywhere, meaning everyone had to work for the glory of the party.
Yet, in a world of scarcity (even in capitalism, resources are not infinite – though much cheaper), prices are the necessary means to distribute goods/resources. And with the socialist calculation problem (nothing is personally owned, so nothing can be bought or sold) there are incredible coordination problems in resource allocation, and end-products cannot be priced, so cannot be allocated based upon need. So e.g. everyone will receive one toilet roll per month, regardless of who had the bad fish for dinner.
The Soviet Union was able to set prices for toilet rolls, tooth brushes, etc. by copying the prices in the relatively capitalist west.
I’m still not sure how the Soviets managed to erect those big, dull, monolithic appartment buildings, when labour was supposed to be free, like toilet rolls (presuming those were available at all); slavery/compulsory labour?
Oh, and tax revenues? Paying taxes requires some sort of an economy, too.