Because it costs money. An https connection requires an order of magnitude more resources to set up and maintain than a plain old http connection.
Even with hardware acceleration it takes more electricity per connection, and dedicated SSL accelerator boxes are not cheap (a pair of load-balancers with SSL acceleration will cost more than a new car).
When you're operating at the scale of Google, that can add up quickly.
Always good to remember that non-techies don't even know what options are available much less how to make use of them.
Aside from that, I'd wonder if in China they are worried about getting caught and getting in trouble with the government.