I believe it is because the establishment, particularly the economic elites, tend to turn a blind eye to fascism because it is a chest-pounding nationalism that has socially conservative overtones. The social conservatives have long been used by the economic elites as a reliable voting block, and the chest pounding nationalists ensure the MIC higher profits.
Socialists, on the other hand, are frowned upon because socialist movements, by definition, put the needs of the people first, as opposed to the needs of the wealthy. Usually, it is the economic elites that have to pay for these programs that benefit the people, and the corporate elites who must pay higher wages, offer better working conditions and extend more benefits.
So, socialists and fascists are, in the eyes of the elite, two wildly different things. Yet, interestingly Bernie's and Trump's respective campaigns are both being painted as anti-establishment campaigns in today's 'news.' I saw Lawrence O'Donnell had a whole show on 'outsiders.' I didn't watch it, and so wonder if he made that point somehow.
The other thing that concerns me greatly about Trump is that the economic elites have said they believe he would be easy to control. Funny - that's what executives at IG Farben and Krupp, as well as the Prussian aristocrats believed about Hitler. Sure, they reasoned, let him take power. We can control him...and we all know what happened there. It cost the world upwards of 80 million lives to 'control' the fascist powers that made up the 'axis.'