I thought Elie's letter states the rationale quite clearly:
I understand their concerns and agree with them that it is crucial that GiveWell has a board that plays a serious role in its governance. After discussing alternative board constructions with staff, current GiveWell board members, and my counterparts at other nonprofits, I believe that the direction we have chosen (which means a smaller board for now) is the right one for GiveWell. In a nutshell, I believe it will mean a more focused, intensely engaged board, and ultimately a more robust, more impactful GiveWell. I recognize there is room for disagreement on this point, and the board will continue to revisit this decision over the coming years. We will expand in the future if we believe doing so will help us further GiveWell’s mission most effectively.
Seems like Givewell had a sense that the current board was large, and this made it harder for the board to serve its function, so they decided to reduce it in size.
You may also be interested to see the thread about this on the EA facebook group: https://www.facebook.com/groups/effective.altruists/permalink/2209495562440122/