Probably has to do with quality of the ladies.  They charge less, so they have less to spend on incall location.  Other areas in NOVA like Tysons and the like are expensive for rooms.

I could be wrong though.

because Woodbridge is in Prince William County where LE activity might be a bit less worrisome.

Zoom in around Potomac Mills on Google Maps and search on motel. You'll see that there are a lot of lower end (i.e. cheaper) places to stay. No-tell motels. You will also find a lot that advertise Springfield, Manassas and Dulles/Herndon I think for pretty much the same reason. The hotels/motels are there because there's lots of traffic through these areas, which also works in the girl's favor. And I'd say a lot of that traffic, a lot of the residents around there are also toward the lower end of the economic scale, which is frequently the target audience for those ads as well. I'm not saying that all of the locals are lower end, nor am I saying that all of the ladies that work out of those areas are targeting the lower end guys or the qv session, but the percentages of those accommodations and customers in those areas would be higher than, say, Tysons or Crystal City.  

It's like the old saying attributed to Wiile Sutton, when asked why he robbed banks he replied "because that's where the money is." Why do a lot of BP girls work around Woodbridge? Because that's where the cheap hotels and quick visit guys are.