Sanders ran on a platform promising to shake up the elites that take wealth away from the people, serve corporate interests and provide limited services to poor people. He definitely ran as an outsider, separate from regular Democrats.
His message resonated well with younger people and I assume that it was more appealing to people who would benefit most from a stronger social support network from the state, i.e. poorer, working-class, people, rather than those who would be taxed more. It also, for reasons I don't quite follow, didn't really catch on with black communities.
On the Republican side, Trump (and some other populists before him like Buchanan), ran by appealing to the disaffection of white voters who have been doing badly economically. This is how he won states which traditionally would have voted working-class Democrats. Globalization can be beneficial to a nation as a whole, through lower prices, but it doesn't mean that workers in affected industries come out ahead.
50-70 years ago, a high school diploma could get you a factory job supporting a lower middle-class lifestyle. Now those jobs have been automated or offshored and CEOs routinely make 300x median salaries. The first two are consequences of free markets and ought to result in more overall wealth, even if not everyone benefits equally and some lose. CEO salaries however are a symptom of a deep malaise in US capitalism and will continue to attract populists like flies.
Trump's position on trade is totally at odds with the traditional free trade and budgetary hawks of the Republican party's intellectual theorists, but it won votes and it solidified his status as an outsider.
Leaving aside the rather dubious premise that a billionaire best known for his flamboyant lifestyle and trademark You're Fired! would usefully improve working people's lot, it is not difficult to see how Sanders' message could resonate with the same people who were receptive to Trump's promise of change and improvement for the "common man", against the "usual elites". The relative lack of enthusiasm for Sander's platform from black and other minorities probably also makes him more acceptable to the ethno-nationalists in Trump's base.
To Sander's credit however, his empathy is considerably stronger. Parts of his program actually made sense. Though I suspect even the parts that did, like universal health care, were too left-leaning to attract enough support to win him POTUS, in the US. That's what made a Sanders candidacy an attractive proposition to Republican strategists.
p.s. I think you'd find considerable variations if you were to compare Sanders approval from modern-day MAGA Reps vs old style Bush Sr free trade Reps.
p.p.s On the parts that probably didn't make sense, IMHO, like the Green New Deal, at least the intentions are good, even if our developing climate change emergency is unlikely to be solved by command-and-control, subsidy-driven, solutions.
