Efficiency... Supersonic is even less green than air travel in general. Efficiency is quite a big issue always when flying is involved. Not that you can't use gliders, but those are not practical for general transport.
It's not even the environment, just the cost of fuel made supersonic travel uneconomical. Saving 3 hours off of a flight isn't worth thousands of dollars to enough people. Boom's innovation is to target billionaires with supersonic bizjets. Personally I think their business model is risky, but not necessarily impossible.