It is because the afferent arteriole, which delivers blood to the glomerulus, has little vascular resistance because it is short and wide. So the pressure decrease is smaller compared to other tissues. And the pressure in capillaries of glomeruli is so high because it is specialized for filtration.

The common sequence in systemic capillary beds is: arteriole-capillar-venule.

The vascular sequence in the glomerulus is different: the presence of a post-capillary arteriole (efferent) explains the postglomerular resistance that increases the glomerular hydrostatic pressure.

Glomerular hydrostatic pressure is more due to short, wide, less resistance afferent arteriole which is essential for glomerular  filtration process.

Hydrostatic pressure in proximal tubules is 10-15mmHg, colloid-osmotic pressure at the supplying afferent arteriole is 27mmHg. When 20% of plasma water is filtered, mean colloid-osmotic pressure in glomerular capillaries will reach 32mmHg. To enable filtration, the sum of both pressures must be overcome, that is 42 - 47mmHg. The difference to 55 is the effective filtration pressure, which is not very high.  

Thanks very much for responses!

Glomerular capillary hydraulic pressure is a better term because the fluid in the capillaries is not static but flowing.  The value for glomerular capillary hydraulic pressure of about 55 mmHg comes from experiments in rats and squirrel monkeys that have glomeruli on the surface of the kidney in which capillaries were punctured and pressure directly measured.  It is assumed to be similar in other mammalian species like man.  There must be some drop in pressure moving down the capillary from the afferent to the efferent arteriole for blood to flow, but it is probably small.  In contrast, the plasma protein concentration and corresponding oncotic pressure inside the capillary rises significantly moving from the afferent to the efferent arteriole.  As a result the net ultrafiltration pressure, which is the difference between the hydraulic pressure (favoring filtration) and the plasma oncotic pressure (opposing filtration) drops from 10-15 mmHg near the afferent end of the capillary network to zero or close to zero (depending primarily on plasma flow rate) by the termination of the glomerular capillaries at the origin of the efferent arteriole. 

Another factor contributing to the high rate of fluid movement across the glomerular capillary wall is the hydraulic conductivity of the capillary, which is significantly greater than other capillaries as well.   

The basis for rhe ultrafiltration kidney shared by all mammals (and to some extent some other vertebrate classes) is a pressure gradient that is high enough to exceed the various other factors affecting filtation forces (internal hydrostatic pressure of the glomerulus, colloid osmotic pressure differences between the blood and the glomerular filtrate etc). This can be seen as the predominant factor that (in evolutioanry terms) has 'set' the mean arterial pressure (MAP) of most mammals being where it is - approx 100mmHg - in absolute terms. It might be suprising for some, but almost all mammals - including mice and humans - have much the same MAP. The exceptions are only the very tallest, the elephant and most dramatically the giraffe, where MAP is considerably higher. This is an obvious evolutionary adaptation to the height difference between head and heart. The rest of the body's tissues in mammals characteristically have a very large drop-off of hydrostatic pressure (from MAP) along their resistance arterioles before the tissue capillaries are reached. Even a slight rise in capillary transmural pressures in the systemic capillaries can lead to localised oedema (extracellular fluid accumulation that exceeds lymphatic drainage). As mentioned by others here, by contrast the resistance arteriole leading into Bowman's capsule offers very little resitance to flow (under normal circumstances) and thus a uniquely high capillary hydrostatic pressure is exerted at the proximal end.

Hydrostatic pressure is high because of the structure of Afferent arteriole (wide and short), increasd resistance offered by Efferent arteriole which allows blood to stay in the glomerulus .

In other words, the blood moves between two arterioles not from arteriole to capillaries and then venules as in the case with systemic capillaries

Hydrostatic pressure is high because of the structure of Afferent arteriole (wide and short), increasd resistance offered by Efferent arteriole which permits blood to remain in the glomerulus

already done 17.10.2014