My best guess is it's a busy road so it's dangerous and not really meant for bicycle and walking. I've done 13 miles yesterday to get a comic book on a bike and this right here is the distance between my house and a friend's house I told I can come on bike because what I just did gave me a feeling I could do it but my ass hurts so not right now. But yeah I want to see how this would play out. Before I would walk but I took a bike to a comic book store because it would've closed if I walk and I ended up getting there in time. Took longer than expected something that should've been an hour probably took 2 or 3 hours. So yeah I can do half of that for sure.
I think it mainly means that Google invests a lot more money in the quality of its navigation for cars than bicycles, meaning that they think it's pretty likely that the cycling directions might lead you into a place where it might not be a good idea to cycle.
Google maps sometimes even sends you down an unpaved road even if it isn't nescesary.
Source: It happened to me several times.