It made sense that King was a leftist-what was the right offering at that time to blacks in America?
The fact is that in those days, right-wing Capitalism appeared to be a proven failure for racial minorities compared to the more left-leaning societies in Europe. It would've been not only silly, but pathetic in exactly the "yes master" way to support the very same establishment and socio-political system that had just claimed for a good 150 years that blacks were incapable of participating as full citizens in their own country.
Note that this doesn't mean "communism is good for minorities!", just that in those years, it would've been breathtakingly dumb to go around supporting guys like Hoover and shouting the "good ol' fashioned American way!". The American system did not work for minorities in those days, and they rightly rebelled against it and looked for alternatives. We might not have been living in a country that has made as much progress in terms of racial equality today if they had not done so.
I salute the civil rights leaders, leftist or not, for forcing the issue and helping to make America a better place to live.