Four reasons I can think of off the top of my head:
1. Automatics have a cut-off that varies, and is sometimes quite short. Taking a nice, long, slow draw is really frustrating when the vapor gets shut off right when it hits the "sweet spot".
2. On the other end of the draw, you need a "primer puff" of 1-2 seconds just to get the coil up to temperature. Manuals alleviate this, since you can just press the button before you start drawing in the vapor.
3. Automatics have flaky sensors that are often activated by wind or loud noise. I was in a club in Branson once & noticed my auto 510 going off in time to the bass drum.
4. Because of said sensor, autos have a hole in the battery section to allow for air flow (to trip the sensor). Overfill your cart or carto, or try to drip, and
juice can run down into it and short the battery. Manual batts generally have a sealed connector on the battery, so you can drip all you want.
Once you get used to a manual, it's really no problem at all. It becomes second nature, and most ppl I know (myself included) find the
vaping experience is *far* superior to automatics.