I'm not really sure why you think Brood War is such a pure example of basic strategy without difficult to comprehend mechanics. Even in your example, recall is far crazier in Brood War. Arbiters can also create a version of a force field on a ramp that's uncounterable and lasts far longer. They're also harder to defend against because they have so much HP.
The fact that arbiters are stronger or weaker than the equivalent abilities in SCII isn't the issue. The issue is getting the AI tech to the point where it can figure out that you have to sac a base because it will waste time sending your army there only to have them recall is going to take a lot longer. An AI can figure out that there is an arbiter on the map, and that means there is a potential for it to recall some units into your base. That's easy.
BW isn't the perfect example or pure strategy, but there are dramatically less game-changing abilities in it. Think cyclone/marauder cheeses or 2 base warp gate allins or ravagers seiging your wall. SCII in general makes attacks come quicker relative to scouting. Injects, chrono, and mules make tech and attacks come sooner compared to BW. Reliable scouting is still basically tier 2.
