There's a pretty large installed base of 4's and 5's and iPads. Iphone 6 plus is the most niche and requires customization. I think premium apps will make it but for many developers, including myself, making a special 6+ version isnt economical yet.
And developers invested into the platform already made their apps for the 4/5.
you don't make an app for the 4/5, 6, 6+, AND iPad. You make an app for the iPhone, and the iPad. Apps aren't seperated between the 6 and 6+. Enhancements made to the app for the 6 apply to the 6+ and vice versa.
There's no CNN app optimized for the 6, and then one optimized for 6+. It's not split four ways.
Or for example, look at the Twitter and tumblr apps. They've been optimized for the 6/6+, but they don't add any new features or layout. All they did was just update it for the resolution, something that you can do in Xcode very easily.
Most developers are most likely updating their apps to provide more information on the screen, I.e. like CNN has. Some are simply just updating their apps for the resolution (Twitter, Tumblr, Alien Blue).
Many developers may be waiting to integrate all the new APIs into their app before they update it completely for the new 6 line. Facebook for example is probably working on integrating all the new iOS features (like extensions, interactive notifications, widgets, etc.) before they push an iOS 8/iPhone 6 update on their apps.
Most developers, if the iOS platform is worth their time, are going to update for the 6/6+.