It's as if big journals are after some drama. Or excitement at least. Not just an important result, but a groundbreaking result in its own right. If it's a relatively small achievement that finishes a long chain of gradual progress, it better be some really famous problem, like Fermat's last theorem, Poinrcaré's conjecture, etc.

I wonder if it's actually optimal from the journal's selfish POV. I would expect it to want to publish articles that would be cited most widely. These should be results that are important, that is, are hubs for more potential related work, rather that impressive but self-contained results.