I think it has to do with rotation of the joycon being more fucky due to technology limitations. This is mostly due to Wii Motion Plus tech being what it is, which is used in Wii U gamepad and 3DS gyro. Wii Motion Plus adds another accelerometer and gyro to help the controller triangulate rotation better. Originally the Wii detected rotation via sensor bar and IR camera. That said the remotes can only detect rotation and motion relative to the object's origin point. So if the point resets while moving then the calibration is off. And Motion Plus often resets rotation more often than general velocity without rotation to my experience. I found this out with ARMS the hard way when you have to punch while rotating Joycon to aim. I could never get the aiming right.
Plus I think it comes down to ergonomics. Holding the controller straight up stretches your wrists with your arms neutral, or strain your arms with your wrists neutral. You also have more control when your elbows aren't tucked by your side. A lot of new Splatoon players mistakes is thinking they need to hold the controller upright and shift left and right to aim. However I find it more comfortable to hold the controller like a big rig or tractor steering wheel which is more horizontal than vertical. You can rotate the controller flat to get your X rotation. Plus you can raise and lower the controller with wrist neutral position for Y. I learn to look with my eyes and aim the controller accordingly to where my eyes look with enough practice. You can also move the controller side to side along with rotating for more X camera control. And the stick is there when you have to about face.
It could be the Joycon's limitation as to why there's no X camera control face flat up since that axis is fucky to read half the time. Also that strain doesn't feel as ergonomic for long sessions gaming since you do have to twist your wrists.