Because decrees, once issued, could not be undone.
True enough.
Even if it could be undone from a legal point of view, it may not have been politically expedient to do so. This was a decision made amongst some very important ministers in the country. To go back on it would have made him look very weak and foolish — and this was in a time when weak and/or foolish kings very often found themselves dead at the hands of others who would like to rule instead.
The Wolf