Why is the name 'propylene oxide' preferred for epoxypropane?
why is 'propylene oxide' the preferred name for epoxypropane?


Propylene oxide is not the "preferred" IUPAC name (PIN): Wikipedia lists the PIN as 2-methyloxirane, which is in line with the 2013 IUPAC recommendations.


  why is epoxypropane the only isomer encompassed by the designation 'propylene oxide'?


It's more of a tradition than anything else. Note how the two ring carbons in propylene oxide are the same as the two doubly bonded carbons in propylene: this is not a coincidence. A very common way of synthesising an epoxide is to oxidise the corresponding alkene (you can think of it as simply adding an oxygen atom across the double bond), and so "alkene oxide" has become a rather conventional way of referring to an epoxide (see also https://en.wikipedia.org/wiki/Styrene_oxide for another example).

So, the meaning of "propylene oxide" is not really "an oxidation product of propylene". It should pretty much be read as "the epoxide derived from propylene", or "the epoxide formed when you add an oxygen atom across the double bond of propylene".

In the strict sense of oxidation/reduction being addition/loss of oxygen, oxetane is an oxidised form of propylene, but there is no simple chemical oxidant that will perform this transformation. So, calling oxetane "propylene oxide" would never have - and has never - caught on.