Because nobody reads serious books any more?
Okay, maybe a little harsh. But hearing "This is a book about child abuse!" will make most people remember the book as a child-abuse book, and will probably make them not read it. And if you had to summarise Lolita in as few words as possible, "child abuse" is easier than "a book about yearning for something intangible and transient which is doomed to disappear but which is the only thing which brings one joy, which is also morally questionable in most people's eyes, makes one question why one is doing any of this, causes deep trauma and hurt on both sides and leaves one dissociated and fragmented, questioning what the purpose of life was in the first place - plus all those issues of exploitation, Old Europe vs. New America, pop culture and monetary transactions, and let's not even get into that whole 'metaphors are stale and don't work in real life' thing, or the amazing literary game of cat and mouse he plays with the antagonist where there are so many intellectual clues and references it makes the Waste Land look old hat".
Do you know who Ralph Waldo Emerson was? I'm curious. I didn't until I was introduced to him by my degree.