When there are multiple sellers in a listing, each seller occupies the Buy Box for only a portion of the time. Ads will run only for the seller that is currently in the Buy Box. As a result, no one seller’s ads have complete data to fully optimize performance. When competing against products sold through an exclusive seller, this puts your listing at a distinct disadvantage.
Additionally, this competition between sellers to control the Buy Box can lead to price-slashing, as the seller with the lowest price is more likely to win the Buy Box. This race to zero reduces sellers’ margins, which reduces their future PO’s to you.
Having a single, exclusive seller negates both of these issues. You can read more about the pros and cons of having multiple sellers in this blog post.