Comment by tzs

I'd guess that part of it is that people tunnels need to keep water out, and drain tunnels want to let water in. Generally keeping water out is a lot harder than letting it in.

There may also be extra safety requirements for people tunnels that could raise costs. A tunnel used for cars for example will need some way to deal with accidents. Imagine a big accident in the middle of the tunnel, perhaps with a fire, during rush hour.

You'll need some way to get rescuers in even if the public traffic lanes are completely blocked both ways by the accident. If there is also a fire you might need a way for people on foot to quickly get out.

These requirements could increase the space requirements for the tunnel, driving up costs. They might also cause limits on where the tunnel can go or how deep it can go, which may drive up costs.