3840x2160 is an easier transition from 1080p than going to 2560x1440 because 3840x2160 is exactly 1080p times four. That means that 1080p TV will not need to be stretched/dithered/whatever when viewed on a 3840x2160 display. Each 1080p pixel can simply be mapped to a 2x2 block of pixels in 3840x2160, so you don't lose quality relative to watching 1080p on a 1080p display. A 2560x1440 display would only have this relationship to 720p. Since 1080p is so common right now, 3840x2160 is more logical since you're theoretically guaranteed to have better picture quality whereas a 2560x1440 display might actually have inferior quality compared to a 1080p display when viewing 1080p content.
Just speculation.