you can get a .bit domain name with namecoins
Then why are all the bitcoin entities (coinbase, bitinstant, mtgox, tradehill, bitcoin foundation, bitcointalk, etc) using ICANN's gTLDs?
because the .bit doesn't use standard DNS service that all web-browsers go through. so unless you have changed a few settings in your web browser, you wont easily get to a .bit address.
and trying to get potential millions of customers to change settings just to visit your site, is a headache for them. so unless you sell someone unique, customers wont visit a .bit website.