Perhaps I trust science too much. I know that science is a provisional enterprise, but it's hard not to take achievements like the computer or the eradication of smallpox as grounds to take a position that science is a very reliable methodology for determining how the world works.
Of course I accept that not all truths are scientific. 2+2=4 is true, but not scientifically true. All bachelors are unmarried by definition, not empirical measure. And no I don't think there's no truth in art. I do mock a priori philosophy, but I quote Hume when I do so and that makes it OK, right?
But on a whole, I think I do put a lot of trust into the power of the scientific method as a grounding for epistemology. Given how much of our modern world is shaped by the products of science it's understandable. So I think it's perfectly reasonable to expect empirical evidence when someone is trying to talk about nature; that conjectures about how things work are backed by more than the mere assertion. If this is engaging in scientism, then scientism is surely a desirable label.
And when the label is used against anyone who uses evidence and reason, the label becomes worthless.