But the question persists: why did the alien invasion novel fail?
I got into writing fifteen years ago. As a kid, I started writing stuff that ripped off whomever I was reading, most of it military science fiction, and thus explaining why I kept working on an alien invasion story. As time goes on, you don't just sharpen your style and voice, but your themes as well. I started to notice that I was interested less in a flat-out war story and more in things related to survival and culture clashes. I still like myself a good war story, but the latter themes seemed to have more depth to them, more substance.
Did my invasion novel touch on survival? Not really. The urgency didn't seem to be there too strongly - the need to live the next day, the next hour, the next minute. Did my invasion novel touch on culture clashes? Not really. The plan was to have at least some of my characters meeting with the aliens and try to talk out their differences, but that was more towards the end of the story. The aliens didn't appear until about a hundred pages into the book, and even then it was one creature who died quickly.
There's no shame in focusing on a handful of themes. On the contrary, doing so helps a writer to prioritize his time. If you think (but have no desire) you should write about sparkly and overly-emotional vampires because they're popular at the moment, then for God's sake, please don't. Don't waste your time on a subject that doesn't hold your interest. If you're bored when you write, you'll be bored on the page, and your readers will pick up on that. Write what you're interested in, weave your own energy and enthusiasm into the text, and your readers will respond well to it.
Maybe one day, if I connect the right dots, I might try tackling the invasion novel again.