PHAWK,

  There are several reasons why for reasons using n-substrate. First
some background,

1) MOSFET transistors are majority carrier devices.
2) N-channel devices have lower rdson per unit area so n-channel
   devices are preferred.
3) In the past, short-channel transistors were created by doping the
    MOSFET body, p-well, and source through a single oxide window.
   The boron out diffuses the Arsenic and forms the body of the n-channel
   transistor.  
   To build this type of structure means that the drain needs to be
   built into a lightly-doped region usually an epitaxial layer. However,
   lightly-doped regions mean high resistance so the device needs to
   built on a highly doped substrate region for example an buried
   collector region or an n-wafer.  
4) The benefit of creating the drain in a lightly-doped epitaxial region
   is that the doping concentration of the lightly-doped region determines
   the breakdown voltage of the junction. So the BVdss, the drain
   source breakdown voltage is maximized.  

   So n-channel DMOS transistor structure is preferred for building high
breakdown devices.

5) In many applications, for example, automotive, high-sided switches
   are used for safety reasons(I've heard). A low side switch is connected
   from the load to ground so the load floats when the switch is open.
   For a high-side switch, the switch is between the source and load so
   the load is grounded when the switch is open.

   If a high-side switch were built in a p-substrate, the substrate is grounded.
   As a result, the breakdown is limited by the substrate to drain breakdown
   voltage. In addition, the current handling capability is limited compared
   to an n-substrate device. The drain's thickness and resistivity are limited
   by the need to build the transistor on a p-substrate.

   So to build high power density, high-voltage, high-sided switches n-substrates
are preferred to p-substrates.

                                                                  Best Regards,

                                                                     Sheldon