Yes, cost was an issue. Also complexity; the documentation says:
"Computers and microcomputers may also be used [for control tasks], but they tend to overcomplicate the task and often require highly trained personnel to develop and maintain the system. A simpler device, designed to operate on inputs and outputs one-at-a-time and configured to resemble a relay system, was introduced. These devices became known to the controls industry as Programmable Logic Controllers (PLC). The Motorola MCI4500B Industrial Control Unit (ICU) is the monolithic embodiment of the PLC's central architecture."
I couldn't find data on how popular the MC14500B was, but I think microcontrollers such as the Texas Instruments TMS1000 were much more popular.
Yes, in hindsight with our computer-centric perspective today, it seems like programming the MC14500 is something you'd get CS and programming people to do.
But the user manual has sections on how to translate ladder logic and similar control logic into a program counter circuit and appropriate code, step by step. If someone was familiar with industrial control and basic digital electronics, I think the manual is about all they would need. The same can't be said for most other microcontrollers which are indisputably full computers, with all of their complexity.
"Computers and microcomputers may also be used [for control tasks], but they tend to overcomplicate the task and often require highly trained personnel to develop and maintain the system. A simpler device, designed to operate on inputs and outputs one-at-a-time and configured to resemble a relay system, was introduced. These devices became known to the controls industry as Programmable Logic Controllers (PLC). The Motorola MCI4500B Industrial Control Unit (ICU) is the monolithic embodiment of the PLC's central architecture."
I couldn't find data on how popular the MC14500B was, but I think microcontrollers such as the Texas Instruments TMS1000 were much more popular.