I thought the book sold ok under Soule. Then Tomasi took over, and, well, the book kinda sucked. Even if you thought Soule's run wasn't that good, I mean, Tomasi's run was remarkably worse.
The Death of Superman-related movies all sold well (I think
Superman: Doomsday still moved the most units), but let's also not forget that Superman movies got shelved for five years largely because
Superman vs. the Elite and
Superman: Unbound absolutely tanked. Of course, I don't think the story for
Elite was all that bad; it got sabotaged by looking and sounding like the most cheaply-made DTV's, and by a long shot (I disliked
Unbound, FWIW, but I can't pinpoint any one reason why it sold so badly).
I think DoS has a lot of appeal to
casuals. I mean, it's not like I'd say that
Superman: Doomsday is somehow a better story than
All-Star Superman, but if you went straight by numbers you'd think the battle against Doomsday was somehow the best Superman story ever told because nothing can match its sales.