From the flow equation, Q = (P1 - P2)/R, where Q is flow, P1 is the upstream pressure, P2 is the downstream pressure and R is the resistance.
MAP which basically represents the perfusion pressure of each organ equals to 2/3 x DBP+ 1/3 x SBP. MAP is the pressure in the aorta using the above equation and corresponds to P1 (upstream pressure) from the flow equation.
As MAP is determined by BP, it is basically affected by CO and TPR. Cardiac output is severely diminished in hypovolemic shock due to decreased preload. So as MAP decreases, P1 drops enough to approach the P2, basically nullifying the pressure gradient and without a pressure gradient, there is no flow.
Underactivation of the baroreceptors stimulates increased sympathetic output, increasing HR and thus CO as well as TPR. By increasing TPR, flow is reduced resulting in increased MAP which functions to maintain perfusion to vital organs.
The balance between maintaining flow and a perfusion pressure is delicate and overactivation of the SANS can eventually increase afterload so much that flow is reduced enough that multi-organ failure occurs from underperfusion.
In all forms of shock, low MAP is what basically causes underperfusion, whether that is a result of decreased CO (either due to decreased preload in hypovolemic and obstructive or decreased contractility in cardiogenic) or decreased TPR in anaphylactic and neurogenic.