Co-founder of Coinbase here
You're right in that it's not the "first US Bitcoin exchange". However, it is the first US bitcoin exchange with some amount of money transmitter licenses (none of the companies you listed have them to my knowledge). If you're not familiar with money transmitter licenses, it's what you need from most state regulators to move or hold money for customers in that state. That's why we can't support all states at launch. You can see a list of the states we support here: https://support.coinbase.com/customer/portal/articles/178054...
There hasn't been a US based exchange which has had meaningful volume in comparison to the larger international exchanges, so our hope is this gives people a US option that is both stable (both on regulation and security) and has reasonable liquidity.