The draft was a routine part of American life and was continuous from 1940 through 1973.
Enlisting for military service used to be a normal part of growing up for American males and there was also a certain percentage who put off enlisting and got chosen for the draft, or who couldn’t make up their minds about serving or not serving and just let the randomness of the draft make the decision for them.
Resistance to the draft started slowly but by 1970 there was an entire infrastructure in place to encourage and aid draft resistors. Very few of them were actually jailed for refusing to serve. Only about 3,500.
The system was ended by presidential decree in large part because it just wasn't working.