Not OP, but for me, it's the jarring-ness of the alarm. A year or so ago, I noticed it's easier to wake up at 6:30am during the summer, when the sun rises and I'm waking up to sunlight. During the summer, I'd usually wake up on my own anywhere from 5-20 minutes before my alarm went off, and it was a comfortable waking up. However, in the winter, I'd be jarred awake by an alarm to a pitch-black room, often smack in the middle of a sleep cycle, leaving me groggy and sluggish.

Once I realized that, I bought a $30 light alarm. It starts to glow softly about half an hour before my desired wake-up time, and except for once or twice when I was too buried under the covers to see the light, it consistently wakes me much more smoothly and pleasantly than getting jarred awake by a jangling alarm. (It has a back-up audio alarm that plays at your normal wake-up time, which shuts off when you turn off the light part.)