Flow is laminar in small vessels because
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
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.