There are, as with every aspect of writing, many ways to achieve success in making people care about your characters. You can make them like them by having them do things that your reader approves of or you can make your character get the reader's empathy by having bad things happen to them and then having them find ways to overcome the bad things. You can make your character active. Readers like characters that try to do things, solve things, stand up when others are sitting down. There are all kinds of ways to create characters that engage readers. Try the above if you haven't.
But I said I was going to make it simple and here is my simple take. And I direct this, in particular, to my fellow discovery writers, those whose process is to discover story, setting, and, yes, character, through the act of writing drafts and not outlines—find your narrators inner voice.
Find your narrator's inner voice. Maybe you will have to overwrite in your first draft a bit for this or maybe you'll have to add in later drafts of your discovery drafts to get the voice down—depending on what kind of writer you are. But what I mean by inner voice is that voice we all have going on in our mind all the time. And when we're not alone, when we're involved in some act, or reaction, it's still going on. It's at this time, in a scene in a novel, that your characters inner voice will be SHOWING rather than TELLING if they're acting and reacting to what's happening. It's the tone and content of these thoughts that will reveal character.
OK, here's the simple: people will care about almost any character if they get to know that character. We can care about some awful characters (Tony Soprano, think Game of Thrones, etc...) if we get to know them. We don't have to even really like a character, just find them interesting and understand motivations. Let that inner voice reveal the character.