Hemp was made illegal due to tireless efforts of a gentleman named Harry J. Anslinger. Mr Anslinger had witnessed how J. Edgar Hoover had managed to create the FBI and leverage himself into running it and wanted to do the same because he had been a prohibition commissioner and prohibition had been repealed and he needed some job security.
So Anslinger propagated horror stories (e.g. Reefer Madness) about Marijuana abuse and got the white folk terrified that all their women were going to be raped by Mexican farm laborers and negro jazz musicians who were high on hemp. He lied under oath to Congress and it all paid off when he got Marijuana taxed (remember the first step to outlawing something it to get a special tax on it (watch out for this with guns and ammo under The One) and got himself appointed as Commissioner of the Federal Bureau of Narcotics (FBN - notice the similarity to FBI) which, like J. Edgar, he made a career of and stayed in the job for 32 years.
One of the first things nylon rope replaced was hemp rope used by the US Military.