In helicopters, why not have electric motors controlling blade pitch?
How would you communicate with (and power) these motors?  

The main rotor is spinning constantly - Wires won't work, they would wrap around the shaft and be shredded. A slip ring and brushes (as used in some electically-actuated propellers) would work, but would also wear away quickly and require frequent maintenance as losing control of a helicopter's main rotor blade pitch is a much more serious situation than losing control of a fixed-wing aircraft's propeller pitch (I believe this captures the consequences pretty well).
Because of the rotor speed balance is critical, so any motors, brushes, etc. would need to be balanced around the rotor hub (either by duplicating the equipment, which also provides redundancy, or by adding dummy weight) - get it wrong and the rotor starts vibrating and may come apart.

The motor would also have to be constantly moving: The swashplate adjusts blade pitch through the full 360 degrees of rotation, smoothly changing the pitch of each rotor blade as they rotate. Duplicating this would require the motor(s) to make constant adjustments as the rotor turns, moving and reversing very rapidly). The level of precision required and the forces involved would likely require high-torque stepper motors and a computerized control system of some kind would be needed to drive them and make the necessary blade pitch adjustments at "rotor speed".



So far the motorized solution has added at least one motor, a slip ring &amp; brush system to communicate with it, and a fly-by-wire computer to read the flight control positions and appropriately adjust the blade pitch through 360 degrees of rotation (at whatever speed the rotor is operating).
That is already a lot of complexity and a substantially increased chance of failure versus the (relatively) simple mechanical solution of a swash plate, and I'm not even going out of my way to think of failure modes (actual engineers designing these things are far more paranoid, and could probably imagine all sorts of failure scenarios which eventually result in a helicopter plunging to the ground).