Is it because the battery life is good and they connect super easily and stay connected? Or is there some other feature that pushes people over the top?
First, there's that Apple "it just works" whole-package experience. If you're bought into the Apple ecosystem, using macOS and iOS, it's really incredibly seamless and gets rid of all those bluetooth hassles. Plus, as you say, battery life is good enough that I've never personally run up against it.
Second, and the part which surprised me to discover after I got mine, it turns out that headphone wires had been subconsciously bothering me. Experiencing earbuds without them is genuinely better in feel, and I find I use earbuds so much more as a result.
I did try some experimental jumping around and shaking my head wildly after I got them, and they didn't slip out from that.
I suspect a lot of this comes down to your ear shape. The fit of the wired EarPods are a decent proxy for how the AirPods will fit. The AirPods are less likely than the EarPods to fall out, because the wires aren't there tugging on them as you move your head, which turns out to be a useful difference.
Yep, that's pretty much it. That and it's so easy to use: Insert into ear, music starts playing on headphones. Remove them, perhaps to talk to someone, and it stops playing. Put back into ear and it starts playing again. No buttons, no bluetooth settings, no setup screen; it just plain works.
I'm worried I would lose the AirPods though.