Is there any reason why a n-type enhancement MOSFET can't be used to swith the postive side to a load?
It can but the gate voltage needs to be higher than the source, by the threshold voltage, in order to turn it fully on, otherwise the source voltage will float at the gate voltage, minus the gate threshold, at the current being drawn.
For example, suppose the MOSFET has a threshold voltage of 4V, at the required load current (which we assume is constant regardless of the voltage) and the power supply voltage is 14V. If the gate is connected to the positive rail, the load voltage will be 10V so there will be 4V lost across the MOSFET. In order to turn the MOSFET fully on, the gate voltage needs to be at least 18V. If you attempted to drive the MOSFET from a 5V, the load voltage would only be 1V.
The problem is, if may be inconvenient to add another higher voltage power supply, just to turn on a MOSFET. For AC applications bootstrapping can be used but it requires additional components. Attached is a simple solution using a transistor but the turn off time is very slow.