The Allies had withdrawn the offer
Napoleon, expecting to win the war, delayed too long and lost this opportunity; by December the Allies had withdrawn the offer. When his back was to the wall in 1814 he tried to reopen peace negotiations on the basis of accepting the Frankfurt proposals. The Allies now had new, harsher terms that included the retreat of France to its 1791 boundaries, which meant the loss of Belgium. Napoleon would remain Emperor, however he rejected the term. The British wanted Napoleon permanently removed, and they prevailed, but Napoleon adamantly refused.