Aerodynamics drives the designs. You can either use a capsule and land using parachutes, or by a winged body. A lifting body is necessary, since larger wing cannot take the re-entry loads. If you have a better idea, the aerospace industry would love to hear it.
While the X-20 is the predecessor, there are a long line of improvements since then, culminating in NASA’s HL-20 lifting body, which the Dream Chaser is based on.
But the biggest differences is that this lifting body will be spaceworthy, unlike its predecessors. And it uses cutting-edge materials that did not exist until recently.
Finally, while NASA is paying for the ability to send cargo to the ISS, the development of the Dream Chaser is directed by SNC, not some middle manager at NASA.