Because too much of SC2's design catered to the progamer crowd that liked that kind of stuff. They made some things easier from an APM standpoint but intentionally added more things to make the have not APM intense.
They really bet wrong on how popular that approach would be.
Yeah, the industry as a whole has been moving away from these types of processes for the last 15 years. There are exceptions where it can still make sense but they have significantly higher risk profiles than video games do.
IMHO, worthless though it is, I don't get why Diablo-likes are called ARPG's. They have lots of A and no RP. (They are Gs tho.)
Because in 1996 experience points, leveling up, character attributes, and magical loot were all closely associated with RPGs. Over the intervening roughly 30 years those mechanics have been adopted by games all over the spectrum. However the genre-name for Diablo-like games stuck due to convention.
Also Diablo being called an ARPG predates Dark Souls by 10 to 15 years.
I played 5 when it came out and didn't think much of it, though it was also controversial at the time. Having it set in Africa means lots of black people - which seems better than pretending Africa doesn't exist?