I'll put it a bit differently though. Setting aside environmental concerns, why would you buy a hydrogen car as opposed to a regular old gasoline car?
Charging from home is a substantial benefit of EVs, performance is another. The Mirai is neither a particularly performant car, nor can you charge from home. Why would you switch to a car with a non-existent fuel network? Even once the network exists, why switch?
Obviously, the environmental issues still exist, but right now that is the only reason you'd buy the Mirai instead of many less expensive non H2 options. The Tesla is interesting even without the environmental aspects.