Here in Tokyo I've not encountered an issue with signal on the metro yet. I had big issues back in London but since moving to Japan 2 years ago it's never been a problem. I've seen similar oversights by Japanese companies when it comes to disregarding or just not really understanding things from a Western perspective so it could be as simple as that.
The London Underground doesn't have 4G/Wifi in the tunnels. As with most things with the tube, the reason is that the infrastructure is very old and installing extra equipment inside those old, narrow and twisty tunnels is quite difficult and expensive.
