The size of the games industry in 2012 when Diablo 3 released was probably at least 10x the size it was in 2000 when Diablo 2 released.
Diablo 2 had sold 4 million copies after the first year of its release. In 2008, 8 years after its release, it was still in the top sellers list for the year.
Sales don’t show commercial success unless you weigh them against the size of the games industry at the time. If you do that, you’ll see that Diablo 2 was much more successful in terms of % of market share than Diablo 3. And as you note, Diablo 3 sold on many platforms, while Diablo 2 was sold on just the PC.
But there’s also the factor of popularity and likability. A lot of people who bought Diablo 3 did so because they were excited for a sequel to Diablo 2, but were disappointed with the product they got and didn’t play it for very long.