Each to their own. I think some books, which others love, are weird, confusing, and boring. (For example, Malazan Book of the Fallen series).
Butcher has a character with special powers, but not too overpowered (though I often wish he had more power), fairly smart (I hate dumb main characters, or lovesick ones), resourceful, has some great friends and contacts, etc. The antagonists are not (at least in the Dresden Files) plain I-want-to-conquer-the-world bad guys. They have different powers. The books have differing plots.
With some series, you know what the next book will hold because they are so similar. With the Dresden Files, the only thing I know is that Harry will probably win at the end (and that, actually, doesn't always happen

).
Butcher's writing was a bit rough at the start, but after a few books it was great.
"...nothing about his popularity makes sense to me."
Actually, I can't understand why he isn't more popular.

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