Personally the cutscene art is what initially endeared me to the game (I suppose I understood and appreciated the storybook intention), but maybe I'm strange. I got the game on sale (I'm not sure exactly when), but if I'd known how much meat there was to it I would have paid more. I got a real steal, and immediately started telling all my friends about it.
It looked like a really good tower defence game (I had fortunately played other examples previously, both simple and complex, and decided I preferred simple), but I had no idea how good a tower defence game could be, so I held off. Silly me. Anyway!
I think appeal is a particularly tricky thing. Price is tricky, but a low price is relatively easy, and people with enough income will gladly pay---but maybe their time budget is a bit lacking. So maybe someone does like tower defense games, but not -as much- as a dungeon crawl or an adventure game, and they don't have time for both. I suspect if I had encountered Defender's Quest for the first time now rather than in 2013, I would be faced with that problem.