Don't know why. I'm baffled.
I prefer manual because I like driving. An auto does it for you. That's not fun for me. Plus it means you're not getting the most out of the engine. Quite apart from auto being a typically thirstier and more expensive to fix transmission too. It's just expensive laziness and less enjoyable. Unless you need a free hand to drink coffee all the time then I'm not sure why you'd prefer automatic.
If you were commuting in a busy city or sat on the m6 car park, constantly pushing the clutch in and out and going from 1st and only to 2nd an auto is far far better. With the driving thing I kind of get but your average driver isn't good enough to utilise the gears on a car good enough for it to be beneficial. Your average family, small car wouldnt benefit from a manual much either and most people don't drive fast sports cars, where a manual may be a benefit. But even then many expensive sports cars are fitted with flappy paddle shifts.
Manuals are beyond over rated, clutches go far more often than a modern auto box and some manufacturers are now claiming that their auto is more economical than the manual.
Driving for fun is a very rare thing now due to traffic congestion and speed cameras everywhere. The manual box will eventually die out.