Because newer versions of kernels add new userspace APIs that can be hard to work around if it's not present. For example, Linux 3.7 added getrandom, which is a

Because newer versions of kernels add new userspace APIs that can be hard to work around if it's not present. For example, Linux 3.7 added getrandom, which is a much easier way to get CSPRNG data from the kernel than /dev/urandom. With regards to Windows, I believe Windows 7 added some multithreading helpers that make mutex support much more performant compared to what you would have to do on XP or Vista. So, no, the executable code is not the same on XP or 11.