Almost 1 in 5 people on the web are still running Windows XP. No version of Internet Explorer on XP supports SNI. Neither does Safari on XP, the browser in Android 2.x, the BlackBerry browser or Opera Mobile before 10.1. It may be a minority of users, but it's not just "people who are still running IE6".
Pretty much everyone would love to use SNI; site owners wouldn't have to pay for extra IPs, hosting companies wouldn't have to worry about securing and allocating them just for SSL use, tons of IPv4 space could be put back into circulation, and CAs would be able to sell many more certs. Unfortunately the group that can't use it is too large to ignore, so none of that's happening.
Pretty much everyone would love to use SNI; site owners wouldn't have to pay for extra IPs, hosting companies wouldn't have to worry about securing and allocating them just for SSL use, tons of IPv4 space could be put back into circulation, and CAs would be able to sell many more certs. Unfortunately the group that can't use it is too large to ignore, so none of that's happening.