They weren't proud of their fame. They didn't ask anybody to publicize Cobain's death.
It's just cause and effect.
Not like they could take it and turn it into cash, really. If anyone profited it was the media and subsequently the record label.
The label's still making a fortune off of their stuff.
The surviving members are benefiting, but each has found success in their own right so it's not like they hyped up his death to start raking it in because it was their last chance to make a quick buck off of the band's name.
Nirvana would have taken off at many other time periods, but when they did it was at the right moment and they just became huge. That lead to mass popularity, which leads to many fans. Some would say that made them overrated, but I personally don't think it's the case. Hobbe it's one of those things where you can like an influence better than the creator, but in the eyes of history the first people who came up with it are the true innovators. They get more credit.