The reason that Microsoft stuck on major version 6 for so long was the dreaded burden that is application compatibility. For all manner of reasons, some good, some terrible, applications often query the operating system's version and make decisions based on what they learn. Sometimes this is legitimate; it's occasionally done to avoid Windows bugs, for example, or to selectively take advantage of new features that are useful, but non-essential. - Ars Technica
So where Apple forces developers to update their apps to retain compatibility, Microsoft does everything they can to appear compatible.