Flow is laminar in small vessels because
The most important determinant of flow of blood in small blood vessels is effective velocity. The velocity is inversely proportional to the cross-sectional area. In the case of small vessels, the cross-sectional area is large. So the effective velocity is low. When the velocity is low, flow is laminar.
Reynolds number is >2000
Total cross sectional area of small vessels is smaller
Diameter of smaller vessels is less
Effective velocity in small vessels is less