Here's why:

* BACs are large: BACs can carry DNA inserts of up to 300 kb, which is significantly larger than other cloning vectors like plasmids.

* Replication control: BACs are designed to replicate at a low copy number per cell. This is important to prevent instability and ensure that the inserted DNA is maintained accurately.

* Stability: Low copy number also contributes to the stability of BACs, making them ideal for maintaining large DNA fragments.

In contrast: Plasmids, another type of cloning vector, are typically high-copy number vectors, meaning that they replicate many times per cell.

So, while BACs are designed to be low copy, their exact copy number can vary depending on the specific BAC and the bacterial host.