Before PLCs were popular, technicians built automation systems using various types of relays. The documentation was done using "ladder diagrams" and the procedure was known as "ladder logic." So the first PLCs tried to take advantage of this knowledge by being programmed the same way since it was already familiar.

Later systems added to this by being programmable with structured text.

The first time I came across a system that was programmed entirely with relays I was completely blown away by what was possible. And also by how heavy, noisy and power-hungry it was for a portable device.