The problem is there are too many unknowns. First, the speed of clotting varies from person to person. There are lab tests that measure clotting time (e.g. INR), especially useful when a patient takes anticoagulants. One respondent mentioned the absence of anticoagulants, but anticoagulants include substances not specifically prescribed to reduce clotting, such as supplements and even food items that reduce clotting. Further, there are genetic factors (i.e. Factor VIII and others) that modulate clotting time, operating independently of medical intervention/prescribed drugs or anything consumed. Age is another facet in the clotting process, as infants often do not clot quickly. Couple that fact with the smaller quantity of blood in an infant's body and it makes sense why some infants actually die from circumcision related bleeding. An infant may retain his penis after circumcision but still die from operation related blood loss.
Additionally, the penis is different than other appendages and extremities. Besides the difference in tissue composition (smooth muscle vs. skeletal muscle elsewhere), penile arteries dilate more than arteries elsewhere and veins constrict more than veins elsewhere (assuming one has generally healthy blood vessels prior to injury). The unique elasticity of penile blood vessels mean that a traumatic injury like penile amputation is more likely to bleed continuously than many other amputations when taking into account the relative area of tissue amputated and blood vessels severed. Other than the femoral or carotid arteries, which, due to their own unique locations, are more likely to cause death from dissection than severing other arteries, the penis is again unique in that the arteries both inside and leading to it do not always clamp off- sometimes, they remain dilated even after a severing injury.
One of the biggest factors of survival (a somewhat controllable factor) in this case is time. The more time elapses after amputation without subsequent medical intervention, the more likely death becomes.
Death from penile amputation can arise not simply from blood loss but also from related issues such as overall stress and pain. Stress hormones and an inflammatory chemical cascade following such a traumatic injury can overwhelm the heart.