The are a number of assumptions in the question that are problematic.

The "monarchist forces" did not win the civil war. The Anglo American forces did.

The North of Italy was not "the fascist part of Italy". It was the part of Italy that was occupied by the German forces after the Italian government negotiated an armistice with the Anglo Americans. The fascist government in the North existed only because of the protection given from the Germans.

Anyway, the main reason Italians rejected the monarchy was that the monarchy was seen as too compromised with the fascist regime. The king had acquiesced too easily to the rise of Mussolini, did not object to Italy entering the war, and intervened too late to depose the Duce.

Moreover, the handling of the monarchist coup against Mussolini and of the negotiations of the armistice were bungled. The Germans did have enough time to reinforce their garrison in Italy, and when the armistice was announced the instructions to the Italian forces were ambiguous.

The king and his government abandoned Rome and fled to the South, leaving most of the country to fend for itself.

So the Italians in the north that were left for two years to suffer under German occupation and the puppet fascist government did feel that the king had betrayed them.