Losing an achievement changes the game’s dynamic and the board position can radically change when that happens.
If you’re so good or wily enough to get a hold of the achievements of Longest Road (2 VP) or Largest Army (2 VP), don’t forget to defend it! 2 points in a game of Catan puts you 20% closer to the win condition than before. Being lax in defending those achievements is really damaging to your game because losing it results in a 4-point swing in someone else’s favor. You lose 2 points while someone gains 2 points. Losing an achievement changes the game’s dynamic and the board position can radically change when that happens.