To expand on JoshH's answer, LabVIEW Real-Time support began on x86 based targets and used Pharlap, now named ETS. ETS does not support PowerPC so in order to support PowerPC based targets, which have advanges over x86 for small embedded targets, LabVIEW Real-Time added support for VxWorks. In NI's documentation you'll probably see VxWorks or PowerPC on those targets, but since ETS is the 'traditional' LabVIEW Real-Time you probably won't find it called out specifically in the documentation.
From the customers point of view, it should make very little difference when you are writing your application. LabVIEW should run and execute the same on VxWorks based targets as it does ETS and even Windows. However, somethings are clearly going to be different such as if you are trying to call external code which would have to be compiled differently. Some of the differences on a VxWorks target are layed out
here. But in general, you can run the same code on an ETS and a VxWorks target it just needs to recompiled for that target in LabVIEW.
Hopefully that clears up any confusion and helps answer the 'why' question.
-JRA
