It's because class war. By its very nature, once that conflict is clear for all participants, we win and they lose. Therefore their main and most succesful effort in the class was was always hiding the fact it's being waged. You might have noticed how in the mainstream there is basically no effort to even medaciously dispute marxism, no, there is a huge effort to hide its existence, distort its terminology and discourage people of researching it in so many ways.