Nobody has guessed the real reason.
Most fliers fly mode 2 without a tray.
If you watch your thumb as it puts in a left aileron it does not travel perpendicular to the transmitter. It automatically puts in a small amount of up elevator. So you have an automatic coordinated left turn .If you turn right, you get down elevator and you have to counteract that.
I bought a Multiplex radio because you can cant the stick up to 15 degrees from perpendicular in order to overcome that.
You can also do that with the new Jeti, But both of those transmitters are quite costly and it puts the trim buttons in an unnatural place.