This probably doesn't hold up, but just throwing it out there. It could be that the iPhone 6 pixel density isn't high enough.
Ever notice that text isn't as sharp when holding your iPhone in landscape mode? (also, when using a widescreen monitor in portrait)?
I'm not 100% clear on why this is, but I believe it's to do with subpixel smoothing, or because most screens are optimised for certain viewing angles - but I'm not sure. Still, I can definitely see a difference between portrait and landscape.
Perhaps they just want to limit extra landscape options to a higher density display which will be smoother.
Again, this probably doesn't hold up - they allow landscape in everything else. But Apple's logic can be very confusing at times!
I agree, I'd like to see landscape homescreens on the iPhone 6.