The velocity of blood flow is the lowest in capillaries because

the total cross-sectional area of the capillaries is greater than the total cross-sectional area of the arteries or any other part of the circulatory system.

Explanation:

Blood velocity is not the same in all areas. In the capillaries it is where there is less speed, while in arteries and veins it is quite similar. This is due to the cross-sectional area of each of the vessels. It is a mistake to think of a vein, artery or capillary individually. We have to put them all together to see that the total area of the capillaries is 100 times larger than that of the arteries or veins. Blood flowing through arteries or veins is going faster because there is less area.

Blood velocity is inversely proportional to each of the areas of its territories. The greater the area, the lower the speed.