In the context of IC chip layouts, the determining factor is almost always die area. Some functions will be smaller on the die in PLA form, other functions will be smaller on the die in microcode form. Since die area is almost always a scarce resource, the chip designer will usually pick the design that reduces die area consumption for any given function.
(puts on chip designer hat) yes and no, as I said upstream the copyright laws of the time did not protect designs, chip copying (stealing) was rampant.
I can imagine that some CPU designers from a particular world view (especially in the 70s) might only design in terms of microcode - but also design tools were pretty minimal, I can imagine that it might have been far easier to design things in terms of microcode rather than as PLAs - what you might be seeing here is the differences between different teams' in house custom tooling
