Not sure if you got an answer already, but the reason is gravity. Gravity pulls the ship down, so you have to move very quickly horizontally so that the curvature of the Earth starts falling away from you. For LEO that's something like 25,000 kph.
If you move slower, you are no longer in orbit and your trajectory will intersect the ground.
If you tried to slow down more gradually, your orbit would keep dropping until you suddenly hit the ground.
Think of it this way: orbital speed is the speed required for a ship to stay in orbit without thrust. If you had infinite thrust, you could land on the ground at any speed you wanted. But without thrust, you have to go from orbital speed to 0 in less than one orbit.
If you move slower, you are no longer in orbit and your trajectory will intersect the ground.
If you tried to slow down more gradually, your orbit would keep dropping until you suddenly hit the ground.
Think of it this way: orbital speed is the speed required for a ship to stay in orbit without thrust. If you had infinite thrust, you could land on the ground at any speed you wanted. But without thrust, you have to go from orbital speed to 0 in less than one orbit.