The skin covering of the glans IS the foreskin. Loss of skin mobility. The nerve endings in the glans are best stimulated by a rolling massage action. Direct friction tends to fire off pain receptors causing irritation and also causes further keratinization of the glans. With the skin system of the penis significantly reduced by circumcision, the mobility is essentially gone and now the penis is a static mass with no dynamic self stimulation mechanism.
Direct friction is now the primary form of stimulation.
So circumcision further reduces erogenous sensitivity in the penis by reducing skin mobility and thus the ability to use the foreskin to massage the glans. We should not forget that circumcision, like any medical procedure, has complications. Men circumcised in adulthood have summed up the overall difference in sensation as similar to the contrast between seeing in color and seeing in black and white. There is reason to think the loss may be even greater for men circumcised as infants, as most of us were. Sexual pleasure continues to be reduced as circumcised men age until, in many cases, we are left with relatively little sensation.