Other than championing human and civil rights for all, equality, the end of government sanctioned and funded inequality and suppression or minorities, non-violent protest, and overall social reform?

Can't think of a single reason.

Because he thought a lot about poverty and how to relieve it, as well as having practical working knowledge of the problems.

I was trying to show that people on both the "left" (MLK) and the "right" (Friedman) advocate for UBI. But thank you for revealing your thoughts on MLK through your question.

His great moral standing and wisdom

MLK is largely seen as a virtuous, wise person. A lot of moral sentiment enters into people's opinions on social economics. Also, people look to group sentiment to calibrate whether a proposal is more extreme than they want to think of themselves/be thought of.

MLK having expressed a favorable opinion about it makes UBI less susceptible to "that's evil, naive communism"-style attacks.