CMOS Second Order Effects
A P-type substrate is typically used in a CMOS process. Without some method of isolation (extra fabrication steps -> higher cost), the nFETs on a die will all share the same body.  Therefore connecting the source of all of the nFETs to the body could cause an undesired connection between the sources of several nFETs.  Take this circuit below as an example:







simulate this circuit &ndash; Schematic created using CircuitLab

In this circuit, both FETs have their sources connected to their bodies. Using a P-type substrate as a shared body would result in M1's drain and source being shorted.