For me personally, it's a combination of reasons. If I consider each reason separately it may not look that great for me, but a combination of those does. Constantly spinning is a good thing, as it helps and encourages to keep some effort and also accelerate/decelerate smoothly. Not that you can't do it on a geared bike, but it feels natural on fixed gear (this connected feeling, my first time riding felt like riding on tiny scale roller coaster). Let's be honest, for most people (including myself) changing gears is no fun, yes, you can keep your preferred cadence, but is that what you always want to get from riding a bicycle at this current moment? Without the need to change gears it's the ride you fully focused on, but it's more engaging and feels natural. Not that it mentally tiring just same focus on the road, all the same, but without "not-so-fun" gear changes. I think, Electronic shifting just proves the theory of "not-fun" shifting, yes, it may be great, but it still need's to be managed and it has it's limitations. Simplicity means it's easier for me keep the bike in good technical condition and also mentally rewarding: I know it's ready for the ride, all I need is to pump the tires and may be wipe a chain and chainrings after a ride. And, it's a new riding experience!