One possible reason is that the beta-testing revealed some bugs and the easiest way to resolve the bugs involved restricting the release version to Win7&8
No, the release notes with the Beta stated that Vista was not supported, Win7 with service pack 1 or Win8. Many people complained about this on the LRBeta forums.
Werner, I moved Silverfast from XP to win7 without any addditional cost when my XP box went up in smoke. Also any program that runs on Vista will run on Wn7 under emulation,
I think that is the correct term.
I think if you run the upgrade you will find about everything will transfer over seamlessly. It did for me when I took my Vista laptop to Win7, drivers, programs, data.... even down to the screen calibration.