When people tell you something about themselves earnestly, you better believe them. That goes for personal relationships and politics alike.
Bernie Sanders has been in the game long enough to understand the profound distinction between democratic socialism and social democracy. The latter’s aim is to use progressive taxation, redistribution and universal social welfare programs to produce a more egalitarian society but to leave capitalism and a market based economy more or less intact – it’s what we have in Scandinavia and I’m all for it. The former wants to bring the means of production and financial institutions under firm government control/ownership, albeit through democratic means, and it has failed miserably time and time again all over the world.
If you don’t believe me you need look no further than Wikipedia:
modern social democrats are more concerned to curb capitalism's excesses and are supportive of progressive reforms to humanise it in the present day.[4][29] In contrast, democratic socialists believe that economic interventionism and other policy reforms aimed at addressing social inequalities and suppressing the economic contradictions of capitalism would only exacerbate the contradictions, causing them to emerge elsewhere under a different guise.[35][45][46][47][48][49][50] Democratic socialists believe the fundamental issues with capitalism are systemic in nature and can only be resolved by replacing the capitalist mode of production with that of socialism, i.e. by replacing private ownership with collective ownership of the means of production
Now, some people argue that Bernie Sanders’ proposels are social democratic in their nature rather than democratic socialist. See for instance this article by Annalisa Merelli.
If that is the case Bernie Sanders has misunderstood a concept he has been using for decades and he is simply giving the GOP propaganda machine a giant boon, for no good reason.
The second possible explanation, and the far more likely one in my opinion, is that he is wedded to democratic socialism because he knows exactly what it is and he sees no reason to change. (We had a very similar case in Sweden some years ago when the leader of the ex-communist party, named Lars Ohly, refused to stop calling himself a communist until it became a crushing political liability.)
It is the kind of stubborn rigidity that you might expect from a radical who is far out of the liberal/progressive mainstream – and who will give the world the unmitigated disaster of four more years for Donald Trump.