The 1.9 series is unstable (mainline) while 1.8 is stable. Classic odd/even unstable/stable versioning.

This is the answer, I had forgotten about this strategy.

As long as it doesn't break API compatibility, why wait?

Because bugs happen. It's likely a change that reaches into the core of the product. Also, would give a psychological boost to upgrading to 2.0, while also subtly warning that the code will be immature until a few point releases are complete.