Complex sentence: Pope Martin V, elected on November 11,1417, is known to be nicknamed "Papa Carnavale": not because he was elected Pope on 11 November, but because he felt that the carnival did not last long enough.
Split into simpler sentences: Pope Martin V, elected on November 11,1417, is known to be nicknamed "Papa Carnavale". This was not due to the fact that he was elected on 11 November to be pope, but because he felt that the carnival did not last long enough.