"Why?" questions are particularly difficult to answer. It is not possible to stare into Johnson's soul, searching for motivation, from our vantage point more than fifty years later; instead we are left attempting to analyze his recorded acts and words, leveraging our understanding of human nature.

However, I believe it is fundamental to recognize that racism acts on two levels in everyone: an emotional level and an intellectual level. At the emotional level racism develops from two deep aspects of human psyche: tendencies to "oppose the unfamiliar" and to "support the group". All of us encounter these tendencies; but they are merely tendencies. As humans we have the intellectual capability to set aside our emotional tendencies for rational reasons.

Scant few of us may have been born in situations where those of a different race are a natural everyday phenomenon. But we are all born with an innate sense of group, starting with the family and successively moving out through friends, neighbours, townsfolk, and eventually, in some, race. When an emotional tendency to favour friends and family manifests itself in word and deed, uninhibited by rationality, that is merely nepotism. When it spreads to a blanket preference for one's race, buttressed by rationalization and a dislike of the unfamiliar, then that can truly be labelled a deep racism.

But in between these I see an everyday racism, an autopilot racism if you will, where the emotional tendencies are simply allowed to wash over one without any attempt made to engage the intellect. It is the "lazy human's racism", of simply going with the flow. It seems to me that Johnson's racism was of this type. As a young ambitious legislator raised in a deeply racist and segregated South, "fighting the good fight" might have made him a better human; but would certainly have sunk any aspiration to a political career. Johnson was no Don Quixote, tilting at windmills; he had bigger plans.

Then in the mid-50's, while Johnson is Majority Leader in the Senate, segregation hits the national consciousness. His long-standing support for the poor and downtrodden suddenly coincides with a recognition that many of those are not only black; but are poor and downtrodden because of being black. At that moment the driven, ambitious, and yes, racist, Senator from Texas steps up to the challenge; confronts one of his own demons; and conquers it. He remakes himself as a Civil Rights Champion.

It seems appropriate to note here that, as President, Johnson was uniquely qualified in a manner that few of his predecessors, and none of his successors to date, have been: as a former Majority Leader of the Senate, Johnson understood the legislative process far better than most presidents. Johnson had perfected that craft in what his contemporaries termed "the full Johnson" (or alternatively "the Treatment"):

flattery, deals, pressure, threats — delivered at all hours in manners ranging from lofty to crude

Further

As Richard Russell, the South’s leader in the Senate during the 1960s, put it to a friend a few days after Kennedy’s assassination: “You know, we could have beaten John Kennedy on civil rights, but not Lyndon Johnson.”

So: "Why did Johnson champion the Civil Rights movement from 1957 on?"

Because it was the defining issue of his time; and, to be recognized as the great man that he aspired to be and saw himself as, one had to not only be on the right side; one had to be a champion of the right side.