IPv4 addresses cost. Most big sites can't quite go SNI-only yet. Soon!
Customer support costs. Why would TLS involve more customer support? Because it's a technology designed only to break connections. Ideally that's only the adversarial connections---but much like the TSA, a mis-designed authentication scheme can cause great stress and drama.
I do think the world will be all-TLS before too long; the parts that aren't will probably not be HTTP over TCP (e.g., content addressable networking).