Likely because out of a large roster, there are many people with different goals going forward. Some may be done and want to quit the game, some may want to raid casually, some may want to raid at a slightly lower level and in a high tier guild, many who aren't burned out will likely still have the goals and mentality that led them to join a HOF guild in the first place. Many probably still want to push hard get HOF and/or push even harder and climb to a higher level still. Trying to tell those people who want to quit, or those who want to keep climbing, let's just raid casually, will not work. So then they depart and you have a half a raid team, maybe less, probably no raid leader, lack of officers, etc.

Top guilds aren't a group of family and close friends, and even if they are, forcing yourself to play with players whose goals, efforts, and skill levels do not align with your own inevitably just makes everyone unhappy. Better or more dedicated individuals will feel held back, weaker or less dedicated individuals will feel overly pressured and frustrated with the expectations of the other players or raid leader. The best way to enjoy raids in WoW is to find a group whose expectations, goals, and capabilities most closely match your own, and a high end guild trying to step back will never achieve that balance because the chances of everyone in the raid feeling that way is slim to none.

Clearly this guild and Pieces felt the best way forward for everyone was to find their own path instead of trying to force everyone's conflicted interests into a single route. I hope they all have the best of luck and all find what they are hoping for on the next step, whether it is on WoW or another game.