First of all, wood has a natural self-dampening property. This is quite useful for designs with long sensor arms because bouncing and the consequent vibration can interfere with reflective sensor readings.
Wood is also easy to work with. For a Micromouse sized robot, even cutting and chiseling a chassis from a solid block is not infeasible! It is easy to assemble a wooden chassis from pieces because you can use glue or screws to hold various parts together.
Micromice utilizing ultrasound sensors should consider using wood for the chassis because of the dampening property. Otherwise, the ultrasound transducer will vibrate the entire metalic/plastic chassis and the sensor will only hear this self-induced vibration.
As for availability, you can find wood practically everywhere. I suggest the use of pressed and treated wood panels/blocks because of the increased density and strength.