Why are no apps updating to landscape orientation for the 6-plus?

Apps like Facebook, LinkedIn, etc. do not support landscape orientation on the iPhone 6-Plus. Has anyone ever heard a justification for why? I use my 6-Plus in landscape orientation 90% of the time w/ a bluetooth keyboard. It is frustrating that very few apps are set up for this. Even the phone app on the iPhone doesn't rotate. Anyone have any insight on this?