You have to hit the heart, spine (and sever it) and that is about it.
Even a blow to the spinal column that does not sever the cord is going to put someone on the ground.
Nerves do not like being struck, and are very sensitive to pressure and impact.
Think of how your hand fails to work correctly after hitting your 'funny bone.'
You have banged the ulnar nerve against the bone.
It does not take much to inhibit nerve function, if only briefly.
If you deliver an impact to the spinal column the person is very likely to fall to the ground from the even temporary inhibition of nerve function.
While the major vessels and the heart in the chest are not very large targets, they are still larger and more numerous than anything in the pelvis.
The pulmonary circulation is only present in the chest.