The one noticeable difference is that the previous two games didn't launch on Game Pass day 1.
DOOM Eternal sold gangbusters because:
1. It was a late 8th gen game, so massive install base
2. Launched during covid, so everyone was at home and bought stuff
3. Benefited from double dipping or a 2nd wind anyway due to 9th gen re-release
4. Ran on toasters on PC
5. Insanely well reviewed and universally critically acclaimed.
DOOM TDA:
1. Not reviewed as well as Eternal
2. Lacks the same installment base
3. While running well, still significantly higher hardware specs than Eternal
4. Dumbass regional prices, so less purchases. 80€ base game?!
5. GamePass title on PC and Xbox certainly nuked some sales
6. Worse soundtrack
7. The coming out of the DOOM Eternal Mick Gordon soundtrack drama.
Sure man. "Just GamePass" was the difference.