ME2 executed on what it was trying to be better than ME3 did. ME2 was massively popular because it combined a sci-fi story people liked a lot with relatively well executed Gears of War popamole, which was very popular at that time. It's like a storybook cover shooter, which is all it was trying to be. ME3 was much less well liked because its story faltered, which even the most cursory glance at contemporary reactions will bear out. Gears of War popamole was also a little less popular by then, which didn't help.