> There have clearly been repeated attempts to implement it.
To implement what? People at the time of Marx asked him what system they should eventually implement. Marx replied he was not August Comte and did not write recipes for the cookshops of the future.
You have conjured up some non-existent system Marx supposedly wanted to put in place, then you say it was a failure, then you admonish people for not seeing Marx's system did not fail. What system? He explicitly said he had none.
Also, China by some measures has the largest economy in the world. Hanging over party conferences and Five Year Plan meetings are pictures of Marx and Mao. How has that failed. "Well, Marx's picture is there, but it's not real Marxist" people respond. Speaking of no true Scotsman cope.