Christensen's book develops a strong body of evidence that incumbent companies almost always lead sustaining technology to market, but virtually never lead in markets opened by disruptive technologies. Desktop PC users don't carry their machines around. They don't need a six-ounce lighter computer. They have a different set of problems. They want MORE storage, even if it makes their machine heavier.
Discontinuous markets start small.
They aren't cash cows in their first years. More importantly, their initial customers don't share key values of established-market customers. Portability does not matter in the value matrix defined by the established desktop PC market. An incumbent firm, playing in an established market, must listen to its established customers. Usually, it's a no-brainer.