They've been trying to get a pedestrian bridge built for ages. They keep having trouble getting permission for it. The tunnel is basically Elon saying, "screw you, I'll find a way around your stupid NIMBY nonsense."

Couldn't have built it over the weekend. You'd have to block traffic on the street to do it. It's a busy street. Also, building a bridge reduces the height of things you can transport down the street. This sounds like a minor issue, but it's a major constraint for SpaceX as they start moving larger and larger rocket vehicles, even for short trips at night through LA to a sea port or the airport.