In order to create a simpler, more elegant, version of Dungeons & Dragons, the designers eliminated most of the situational modifiers that appeared in earlier editions. See “How D&D Next moves toward a simpler core game.” While these modifiers appealed to players who favored simulation, they slowed play and were often forgotten. Besides, simulation has never been D&D’s strength.
In fifth edition, when someone gains an edge, they gain advantage. When someone suffers a handicap, they suffer disadvantage. Most of the rarely-meaningful and frequently-forgotten pluses and minuses disappear.
But as the designers worked to purge situational modifiers, D&D, thieves tools presented a problem. In earlier editions, Rogues used thieves’ tools because they granted a +2 bonus thievery checks. But this new design had no room for that +2 bonus. Still, thieves’ tools have appeared in equipment lists since the early days. I’m certain the designers felt compelled to keep the tools in the game. But without a bonus, why should a rogue bother spending for the toolkit?
The designers arrived at an ingenious solution: tool proficiencies. By making the use of thieves’ tools a proficiency rather a skill, rogues still need to buy the tools to pick locks and disable traps.
When the designers worked so hard to eliminate the +2 for tool use, why did they bother preserving the +2 and +5 bonuses to AC gained by cover? These bonuses stand as virtually the only situational modifiers in the game. Why not just give disadvantage to someone attacking a target behind cover?
The modifiers remain because they combine with disadvantage. Multiple instances of disadvantage do not stack. If you suffer disadvantage from two sources you still only have one disadvantage. So if an archer suffered disadvantage because she targeted someone behind cover, and if she also suffered disadvantage from long range, she would still only suffer one disadvantage. The fifth-edition designers favored simplicity over simulation, but they weren’t ready to make hitting someone behind cover at long range exactly as hard as hitting someone just at long range.