Why might have all of the racial segregation/discrimination laws of the south come back to be called "Jim Crow" laws?

The term Jim Crow was widely used as a euphemism for black persons, from the color of the bird. The 1832 faux-minstrel song "Jump Jim Crow" (a white portrayal of a slave stereotype) was used to attack Andrew Jackson's populist policies.

So the laws that were enacted following the easing of Reconstruction were called Jim Crow laws. While they could not openly prevent economic activity by African-Americans, they established a legal system of second-class citizenship by requiring blacks to use separate-but-equal facilities (whose blatant inequality was often overlooked). Notable were "back of the bus" public transportation rules, and refusal by white restauants to serve blacks (Negros, colored) at the same lunch counters as whites.

Another sub-genre of Jim Crow laws outlawed interracial marriages (miscegenation). These were widely enacted in the North and South.

It was not until the period after World War 2, when the armed forces were finally integrated, that the move for civil rights gained impetus, culminating in violent clashes in the South as voter drives and demonstrations became more of a threat to the white hierarchy. The landmark Civil Rights Act of 1964 was the beginning of the end for racial discrimination disguised as a legal public policy.