When King Richard III rode towards his fatal clash on Bosworth Field (or Redemoor, as he'd have known it), all bets favored him. He held the kingship, and along with it the ability to confer wealth and power right now. His great-grandfather may have been able to command loyalty as his birthright, but Richard could buy it.
The actions of his father, brothers, uncles and cousins had eroded the rest. Yet he'd been right alongside them, playing his part too.
By 1485, real power lay with the nobles. Those who had risen to prominence by backing the right people at the right time, playing dangerous games successfully for the whole three decades. These were the men with the titles, wealth and land. They could command huge militia forces, experienced after all of those years of battle. They could decide whom they would favor in the next clash.
Richard still had the edge. He was known. His family had ruled for centuries. He had been born to govern vast estates, and had grown up alongside monarchs, watching, advising and finally becoming one himself. Politically it made sense to support him. Monetarily, it certainly did.
So when the call went out, most nobles answered it on the side of their extant king.
Henry Tudor was the opposite in all things. Untried on a battle-field, inexperienced in war, exiled in Brittany for half of his life. The first half had been spent in deepest Wales, a fact always subject to suspicion in 15th century England. He didn't have the personal wealth to purchase influence. Any titles and land on the negotiation table amounted to nothing more than promises only. He didn't have the power to confer the reality.
But he did have a formidable mother, and Margaret Beaufort was married to the most successful noble of all. Thomas, Lord Stanley, had reached that level by consistently picking the right side - or, more usually, hedging his bets with a brother on one side and himself on another - and his decision at Bosworth could make all of the difference.