Most secondary routes in Virginia aren't important enough to be trunk, as the ones that were in the past were added to the primary state route system. I don't actually think there are any secondary routes tagged as trunk besides from LCP currently.

LCP doesn't really have much along it, other than some SFH subdivisions and office/industrial parks, whereas FCP and PWP link town/city centers. LCP also doesn't really outstand from the other divided arterial roads like it in the eastern Loudoun area. It isn't high-speed like FCP or PWP are for most of their routes, and doesn't serve as an intercity/transregional connector like the other two parkways do. 

Traffic along LCP will mostly be local traffic whereas a trunk route like VA 28 or US 15 will be carrying traffic that could be travelling between Manassas and Frederick. Though it was not built to be one, it functions like a business route of VA 28 like US 1 is along I 95, US 29 is along I 66, or VA 236 is along I 495, despite it veering away from it near the DTR.

The most important local roads should always be primary. The trunk network is for non-freeway highways that serve a similar purpose to what an Interstate does, and for that reason, I really don't think that there should be any trunk routes within the box of VA 7, US 15, US 50, and VA 28, at least for the time being if something like the Bi-County Parkway is built.