There’s a selection bias in that the USSR and China both actually turned into barely functioning societies afterwards, often because they implemented their ideals in inconsistent or hypocritical ways. If you take the same ideology and actually apply it consistently you’re the Khmer Rouge.
Sounds similar to religions. If a religious group sticks strongly to its religion's founding principles and teachings, it's "fundamentalist" and is basically a cult or something like The Handmaid's Tale. The groups that water everything down and are hypocritical and inconsistent are much more successful long-term, with far more members and lots of money.
The main difference is that unlike Marxism, most traditional religions can be the basis of a long-term successful society even if you apply their ideas consistently.