There are a couple of reasons reasons that are commonly cited:
- The blockade was an act of war by the Western Allies against Germany, and the state of war against Germany existed until the Treaty of Versailles was signed. (Keep in mind that an armistice is just a cease-fire.) Keeping the blockade was one way of forcing Germany to stay at the negotiating table.
- The blockade was a cynical way to starve the German population (quite literally) as an attempt to keep the German revolution contained within Germany, and to channel it towards a resolution in a German republic. (In other words, it was in the interest of the allies to quell both extreme left-wing and right-wing factions fighting the republic.)
- The blockade of food supplies in particular was unnecessarily extended due to German reluctance to use its merchant ships to load food from Allied ports for import into Germany, which the Allies offered in January 1919. The Germans feared that should peace talks break down, its ships would be interned or confiscated in Western ports.
I think the truth is likely a mixture of all three of these, with a fair amount of bureaucratic inertia thrown in. If you're interested in gauging the effects of the blockade directly, here's a free resource: https://libcom.org/history/allied-food-blockade-germany-1918-19-n-p-howard