How can PMOS eliminate body effect but NMOS doesn't?
It works when the source is at the same potential as the substrate.  But not all NMOS transistors will have their sources connected to the substrate.  Sometimes they are put in series, where the lowest one is connected to the substrate, and then the drain is shared with the source of the next transistor and so on.  It's also possible to use an NMOS in a transmission gate.  In this configuration, both of the source and drain are connected to signals and neither can be connected to the substrate.  

Also, connecting the PMOS well to the source will significantly increase the capacitance between the source and the substrate, which could be a major factor for speed.