Why were the reserve clause and free agency such hot button issues for so long in professional baseball?

These were hot button issues because the reserve clause
represented the old way in which owners of sports teams completely controlled their
players while the idea of free agency was what the players wanted as a way to assert
their independence and to make more money.

Before Curt
Flood challenged the reserve clause, players could not move from team to team without
the permission of the owner.  This meant that the players essentially had no way to
force owners to pay them commensurate to their worth.  The owners would simply dictate
what salaries the players would make and the players had not choice but to take
them.

The reserve clause was a symbol of an older, more
patriarchal America where authority was always obeyed.  Free agency was and is a symbol
of a more modern America where everyone pushes for their rights.