Lots of thread's on this already with answers ranging from "they aren't good at anything, I'd rather use a gyuto" (somewhat valid) to "its a woman's knife" (absurd).
I think in general its because we're hobbyists and optimizers. There is nothing a santoku does that another knife - petty, nakiri, small suji - couldn't do better so most people want just the right tool. On the other hand, I quite like mine, and not just because it is top tier steel. Its great for nights where I want to quickly break down a chicken, cut a few veggies, and chop some herbs and alliums with just one knife to clean up. Could I do that better with 2-4 different knives, duh, but the santoku works well enough.