The Holdo Maneuver has a really, really simple explanation. The only reason the ships collided is because of the hyperspace tether that the First Order used on the Resistance ship. Without the tether, the interaction would only have destroyed the Resistance ship (it's established elsewhere that you can die in hyperspace if you collide with a gravity well, which is why ships have safety mechanisms that drop you out of hyperspace before that happens; that's how an Interdictor works). The lack of a tether is why you can't use the Holdo Maneuver in every other space battle.
The other key element is that it was new tech, so neither side knew with certainty what would happen. This explains why Holdo was so cryptic about the plan: she didn't know if it would work or not. Turning off the hyperspace safety measures and trying it out was a last ditch "well, we're dead anyway, why not give this a try?" tactic. And it explains why no one in the First Order responded fast enough to stop her. Hux (and others) only realized what might happen at the last minute, which also explains their reactions.
Going further, this also explains why we don't see hyperspace tethers used again. People now know that if a capital ship uses one, it puts a huge target on their backs; safer to stick with the old-school Interdictor style gravity wells. You could, however, have a suicide squad sneak a hyperspace tether onto a ship or outpost specifically to employ the technique.
