Why does an obstruction in the proximal convoluted tubule decrease glomerular filtration rate?

Obstruction in PCT ( proximal convoluted tubule of nephron) will increase capsular hydrostatic pressure hence there will be decrease in effective filtration pressure. That will affect GFR adversely.
To understand this we must recall that functional component of kidney the nephron is associated with a tuft of porous capillaries (named glomerulus) at its blind end i.e. near Bowman’s capsule.
The Blood Hydrostatic Pressure in glomerulus is increased by narrowing the arteriole draining out of glomerulus . You must notice that diameter of efferent arteriole is less than that of afferent arteriole.
In glomerulus blood overcomes its own Osmotic Pressure (due to presence of plasma proteins etc) so that filtration can take place. Water along with urea salts glucose etc. leave blood and accumulates as Glomerular filtrate in Bowman’s capsule.
The Capsular Hydrostatic Pressure of filtrate that remains present in the Bowman’s capsule also counteracts on high Blood Hydrostatic Pressure generated in glomerular capillaries.
Thus Effective Filtration Pressure is much less than the Glomerular Blood Pressure. The EFP helps in producing 90 to 120 ml of filtrate per minute in kidneys: this is called GFR (glomerular filtration rate).
If there is a blockage in PCT of nephron filtrate will keep accumulating in Bowman’s capsule. This in turn will further increase Capsular Hydrostatic Pressure. Net filtration Pressure will decrease. Hence less amount of filtrate could be generated by glomerulus per unit time.
In brief we may say blockage in PCT will decrease GFR which means filtration of blood will be hampered.