I, personally, am drawn in by character, character, and character. Flawed, sympathetic, three-dimensional people with desperate needs and a fire lit under their asses. People I can understand, like, and empathize with. People I get worried about. It's like making a friend. I have to know them - and like them enough - that I give a shit if they live or die.
That's the "why" I'm looking for. Right now I could give less than a shit if some made-up country is over-run by goblins. Simply being a massacre is not enough to catch my interests - I'd rather spend my brain energy on the real-world massacres in South Sudan actually going on right now. The actual, real, literal world we live in is full of genocide, human trafficking, disease, and all kinds of shit I could (and SHOULD) be spending my time worrying about. Why would I, as a reader, want to pick up a book in which someone made up a bunch of shit about human suffering in an alternate universe?
Answer: because it's happening to a character I give a shit about.
This is why I'm more likely to waste time helping that friend who's roommate attacked her move out of her apartment asap, or take in the wayward runaway at my door, or testify in the court case of the single dad who's babymama is a total crackhead. Because they are real people with real emotions in my face and I give a shit what happens to them next. Every one of them is having a first world problem. Everyone one of them has clean running water and health care (to what extent we have health care in america) and isn't literally being forced into prostitution at gun point, starving, or having bombs go off around their houses. Hell, I'm going to help the one friend without a car get to the apple store to fix her phone because it's cold out, speaking of first world problems, instead of concerning myself about South Sudan and Syria and Darfur. Why? Because I don't actually know any South Sudanese. Yes, people are dying, like they always do. But my friend with the broken iphone is really upset and I like her.
That's just how human motivation works - we are only capable of caring about so many people, before it becomes a statistic. I can flip on the news any day to hear about someone getting shot in right here in Chicago and not be nearly as upset as if one of my friends was the one who got shot. Why? Is that person across town less human than my friend? Do they count less? No, I just don't know them. There was a study I read somewhere recently (I can probably google it up if you're curious) that says the human brain is hardwired to have a limit of about 130 people it can care about - the maximum size of a tribe in a hunter-gather society, the context in which we evolved for millenia. That's how big "us" can possibly be, before everyone else is "them." I mean DEEPLY care about - the way a person is wrecked for months, years, or the rest of their lives if their spouse of child dies, not the way people calmly say "OMG, I'm so upset" when a celebrity dies and forget about it in a few days.
To make me give a shit about your plot and whether or not goblins overtake a fictional country in a fiction world, you have to make the people of that country part of my tribe. Part of my 130 people I am capable of giving a shit about.I have to get to know at least one person, vividly, and like them, and worry about them.
They have to become my friend.