Why is it that most pictures of MLK Jr. are in black and white rather than color?

Up until the 1970s, and still, well into the late 1980s, the preferred film stock for photojournalist was black and white (B&W). B&W film was THE tried and tested film of choice for photographers. It was predictable, meaning that the film stock would roughly reproduce the same accurate image. Color film stock needs much more light and other variables to have a flattering image (still many would opt-in to certain stocks known for great colors). B&W film was also, simply, the only mass-produced film stock for a very long time. Color film photography was also seen as amateurish or for advertisement photography.

Keep in mind that B&W was typically cheaper than its color alternatives due to the various forms to develop the film as opposed to color that can really only be developed with C41 chemicals. Simply B&W was the most widely available and most well-respected way to do photojournalism or even at-home photography (if one could afford a camera).

Now, most of the photos we have of Martin Luther King Jr. are from photojournalists covering the civil rights movements. Due to the aforementioned statements above, the very nature of photography and photojournalism at the time was in B&W. While cases exist of color photography, mainly the famous (and my favorite historical film stock) Kodachrome, it was much more of an aberration or creative choice. It was really not until the street photographers of the 1970s and 1980s that popularized color film we see much more color in photojournalist reporting. (Here's Alex Webb describing his shift from B&W to color in the 1970s).