Just because it's not very nice? Especially uphill with four lanes, traffic islands and vehicles constantly parked on the left, and speeds of 50mph or so.
Try riding side by side having a conversation up the A70... not so much. WoL would be much more civil.
I don't mind a good belt along the main roads to get my exercise to and from work, but I don't like it enough to do it unnecessarily or inflict it on SWMBO.
It's pretty interesting stuff. I think we're clearly not car dependent since between us we do 150+ miles a week by bike, but then again, the fact that I can utter the words "second car" (even as a remote possibility) when we live *on* the WoL path might suggest otherwise :)