Because reducing is far more complicated and risky, if possible at all. It needs complex operations inside the guest, and depends on all the layers you use (file system, volume manager, software raid, crypt layer etc...) in the correct order. Most of the time it can only be done offline. And sometimes, it's just not feasible (eg, xfs filesystems can't be shrunk). And of course, any error during all those manipulations is a risk of loosing all your data.
