Not a surprise, 2019 didn't have a single 'bona fide' RPG that wasn't critically flawed in some way. Worth remembering last time we had one - Pathfinder - it won handily.
Well, I wouldn't call Pathfinder, not critically flawed.
Bugs, RTWP, time limits
Not to mention Codex GOTY Divinity: Original Sin 2.
This is my take: There are a lot of people who vote in our GOTY polls that are perfectly fine with enjoying the mainstream stuff (not that there's anything wrong with that!). That's why poor TOW at the bottom of this year's list has 50% positive reviews as well, given to it by a large number of voters.
So when you take that critical mass of mainstream game lovers, and give it an extra edge - turn-based combat, authentic oldschool appeal, particularly good writing, etc - you get a game that's very likely to win #1. Even if it has flaws.