Because Angular 1.x has a long term support plan in place that should see maintenance provided to the framework for at least a couple of years after Angular 2 drops. Another major component of that plan is building out a compatibility layer between Angular 1 and Angular 2.
For more information on that plan, see here: https://docs.google.com/document/d/1xvBZoFuNq9hsgRhPPZOJC-Z4...