Because I can’t stop thinking about the scene:
One theory is that it takes more space for a ship to accelerate to light speed than the distance between the Raddus and the Supremacy, so it would only be going a small fraction of the speed of light at impact. Of course, that doesn’t fix the problem of weaponizing the hyperdrive.
So I imagine Hyperspace as a different dimension entirely, and ships accelerate along a non-Euclidean vector causing them to only weakly interfere with normal matter. So, most of the Raddus’s mass was already in Hyperspace at impact, making it much less violent.