Clogging up the rail grid was somewhat acceptable when it was a few end-of-line terminal stops, but now those tunnels are in the middle of the rail network. A bus breaking down and blocking the tunnel was bad enough when it affected end-of-line service, but would be an absolute nightmare when it affects middle-of-line service.
Sorry, downtown single-occupant vehicle drivers, you're just going to have to deal with the consequences of spending tens-to-hundreds of thousands of dollars on your choice of the least space-efficient, gridlock-inducing form of transportation.