That is certainly true. However, the opportunity structures they face change quite dramatically over time. There were thousand of religious dissidents and 'heretics' in European history - Martin Luther was just one of them. What made him "special" or rather what made the movement he started so effective - was the unique opportunity structure created by the growing merchant class ready to burst asunder the integument of feudal Catholic religiosity. Ditto for Marin Luther King.
Social movements and culture in general are like a garbage can. People put there anything they want, other may scavenge through it for what they need, but most of it goes to waste anyway. However, sometime someone picks up something from there and succeeds in turning it into gold - simply because he or she was the right person, at the right time, with the right message with access to the right dissemination channels. Without that opportunity structure - the messenger and his/her message would end in a scrap pile, like thousand before and after him/her.
But it is also equally wrong to assume that the messenger and his message were just a product of impersonal "econo-historic forces," and if it were not him/her then someone else would step into his/her shoes. Just as thousands of messengers ended in a junk yard of history because they did not find the right historical opportunity, an equal amount of historical opportunities ended in the same place because they did not fight the right messenger.
Wojtek