They didn't. Read also the sources. It's true many NS people use it, we disagree among eachother here but as metapedia says this is our internal consensus based on facts such as Hitler never said such "shortening" in a single of hundreads of his public speeches, he pronounced lengthy 'Nationaler Sozialismus...'. Not single official document exists. They were National Socialists and NSDAP party. Yes it's common shortening. By jews.