Short answer: Because you only use one hand on the yoke.
Longer answer: In an airplane, you almost always fly with only one hand on the yoke (or none, as the case may be.) You will be using your other hand for power adjustments, setting flaps, adjusting radios, setting instruments, writing, etc.
In a car, power is adjusted with your feet, so you don't need your second hand for power adjustments and there aren't really any other particularly critical controls that need to be manipulated frequently or precisely. However, in an airplane, you have to be able to precisely control three axes of rotation, not just one like in a car. As such, your feet are needed for yaw control (and differential braking,) which means power must instead be controlled by hand. So, there's no need for the various grip positions that you could use with a wheel because you don't want your second hand on the yoke in the first place.
Additionally, having a 90-degree angle (or nearly so, as the case may be) allows for a better grip than the smooth curve of a steering wheel. You can place 2 or 3 fingers above the horizontal bar and the others below, which prevents your hand from slipping up or down the control wheel, as is possible with a normal car steering wheel.
As far as the second question of whether it would be better to have 180 degrees equal full deflection, that would mean you would either need 2 hands on the yoke in order to command full deflection, which is not good for the reasons already mentioned, or have a weaker grip on the yoke that would allow you to rotate it 180 degrees either direction. If you have a firm grip on the yoke, your wrist will simply not be able to rotate enough to turn it upside down without releasing your grip.
It's also important to note the difference here between what the yoke controls versus what a car steering wheel controls. A car steering wheel directly controls your turn rate. Thus, a car whose steering wheel is in the neutral position will not be turning. In an airplane, however, your current bank angle controls your turn rate. An aileron deflection changes your bank angle. Having the ailerons in the neutral position will not cause you to fly straight forwards, but rather will simply leave your bank angle (and thus your turn rate) at its present value (if we ignore the effects of dihedral and such.) Thus, you don't need as much possible input control range to get precise turn rate control.