It was suggested that they should build a tunnel, for safety reasons. They built a bridge because it was cheaper. Bridge detractors speculated as to the date the structure would come crashing down, thanks to this type of incident. At just under 50 years, it’s had a good run.
There was already a tunnel - Baltimore Harbor tunnel that opened in 1957.
I meant to also post the link to the Wikipedia article: https://en.wikipedia.org/wiki/Baltimore_Harbor_Tunnel
They built a bridge so Haz Mat vehicles had a route. The existing tunnels are a no go for such trucks.