I can’t really speculate on that, but I can say the PDP series is a dream to program. The instruction set is simple and orthogonal, and you can hold an accurate model of the whole machine in your head that tells you exactly, step by step, what will happen when your program is executed. I don’t know if any other contemporary machines also had these properties, but I do think it makes it an ideal research machine for creating a new programming environment (and that’s really what UNIX was initially). That makes me think it was very likely either the PDP or a similar machine would end up at Bell Labs for people to develop on.

I think I learned -7 or -8 assembly in my comp org class. It had about 8 instructions and you had to make your own division instruction, and I think multiplication.

Then we moved onto a bit of 680x0. Yeah, that was different.

A simulator for it was pretty easy to write, switches and all.

I learned on a PDP-11, but we had 8’s available, too. They felt very similar. I think the 11 had maybe 15-ish instructions?

Edit: just checked, and the 11 has about 70 instructions. I think MACRO-11 might abstract over many of them, making it look like it has fewer instructions. The instructions group nicely into a few families, which makes things really easy.

There isn’t a single pdp series: there are 18 bit machines (pdp8) 36 bit machines (pdp10) 16 bit machines (pdp11) with very different instruction sets