Because small needles cause pressure changes in the collecting tubes and may cause hemolysis or damage to red cells. This hemolysis can cause a specimen to be unacceptable or worse yet, when it is small enough to get by, will cause incorrect answers. So, the standard size needles became standard because they cause the least amount of trauma to the patient while giving the laboratory an acceptable specimen. When blood is drawn from a vein in your arm, the cells are still alive and that cause changes that must be dealt with. Prolonged time in transit can cause false result.
Some test tubes have chemicals that separate cells from plasma to prevent contamination.
Others have compounds that interfere with the cells' function. Some tests must have their specimens chilled on ice immediately while others requires heat. Jiggling the needle or other such maneuvers may also increase contamination invalidating the result. This is a poignant problem when trying to draw blood from a struggling and terrified child. This is probably the most frustrating preanalytical issue for the laboratory.