A number of reasons for that with the most important being directional instability any time the wheels are touching the ground. The reason for this is the Center of Gravity of the airplane is behind the main gear contact point with the surface and any time the airplane is moving on the wheels that CG will try to swap ends with the wheels. With a tricycle arrangement and the CG in front of the main gear the CG will want to stay in front of the wheels, In other words will remain directionally stabile.

Another point is with conventional gear (taildragger) if the gear is mounted to far forward The airplane will tend to bounce on landings much more. However if its to far back the airplane will tend to nose over. So its important to hold full up on takeoff to a point and release much of the up or the airplane will jump into the air to slowly. Some airplanes this is more important than others and is very dependent on landing gear positioning. This is one consideration that flyers of tricycle gear don,t really think about much.

John