AG Automatic Failover using FCI

Hi,

I'd like to understand why it is not possible to automatic Failover Availability Groups using Failover Cluster Instances. I think it would be great for DR and HA. Do you understand why that limitation exists?

The link https://msdn.microsoft.com/en-us/library/hh213242.aspx says: 

• SQL Server Failover Cluster Instances (FCIs) do not support automatic failover by availability groups, so any availability replica that is hosted by an FCI can only be configured for manual failover.

Thanks a million,

Rafael