Why does a single RC network does not provide 90 degree phase shift?
A single RC network will provide 90 degrees in the extreme but the problem is that the amplitude of the output will be zero.

Consider a 10kHz voltage feeding a grounded 10uF capacitor - the current in the capacitor leads the voltage by 90º as you'd expect - this is always the case with capacitors. I always remember it by CIVIL - in a (C)apacitor I (current) comes before V(oltage) and V(oltage) comes before I in an inductor(L).

So if you have a 0.1 ohm series resistor feeding the capacitor the current will be less than 90º leading - probably between 85º and 90º at 10kHz (EDITED from 89º). As the resistor value increases so the angle of the current reduces back towards 0º.

If the resistor were 10kohm, it would be defining the phase angle more than the capacitor and the phase angle would be closer to 0º.

So it's theoretically impossible to provide 180º phase shift with two RC networks. Making one RC network provide 60º is very easy and cascading three does the job.

Does this make sense?