"The Masque of the Red Death" is about a "voluptuous" masquerade ball, and somebody shows up dressed like a victim of the Red Death…and it turns out to actually be the Red Death. But seriously, it is a pretty catchy title, isn't it? Don't you want to read on and find out what the "Red Death" is when you see it?
You might find it interesting to know that Poe's first title for the story was "The Mask of the Red Death." That puts the emphasis on the mask/costume that the Red Death wears to the ball, rather than the ball. Itself. We prefer the second title, even if it's only a matter of a letter or two. Not just because the "q" makes it look more interesting, but also because we think the masquerade itself is what makes the biggest impression on the reader in this story. It makes sense that it should be featured prominently in the title.