Safety reasons-- Bunkers are designed to be played out of under the Rules of Golf(contrary to water hazards--that is why Rule 26 allows relief from WHs under a one stroke penalty)--
Players in bunkers are therefore entited to relief from Immovable obstructions(planks, bridges, steps, etc) in bunkers so long as relief is taken in the bunker--Rule 24-2b(ii)(a)-- Of course if the player wants to take relief outside the bunker a new Rule change this year, 24-2b(ii)(b) allows the player to take relief outside the bunker in getting relief from an IO under penalty of one stroke(same as the WH rule)--
Finally, the Committee, if it believes course playability overcomes safety concerns, can make the obstructions an intregal part of the course, in which case no relief under Rule 24(the player can declare unplayable under Rule 28 and get relief from the plank, but he pays a stroke penalty AND must take relief in the bunker(unless he takes relief under stroke and distance -- 28a))