Why do they use the lower co borrower credit scorwe?

Your lender will pull a "Tri-Merge" credit report, which includes 3 scores from Experian, Transunion and Exuifax. They will look at the middle score, and if there is more than one borrower, they will use the lowest of the middle scores. This is standard practice for all mortgage lenders and mortgage programs.

The federal agencies that oversee mortgages, like Fannie Mae and Freddie Mac, require lenders making conventional loans to focus on the lower of the two FICO scores. It all has to do with the risk. The lower the score, the higher the risk. Usually it is the lenders that will price the rate, or fees higher for someone having lower scores.The actual approval can happen regardless of the score, but when it comes to your terms, they always look at the lowest middle score of 2 borrower.

they use the lowest middle of all borrowers.

They use the "worst case" risk, because the ultimate investor, such as Fannie Mae, requires it.

In a 2-borrower situation, we are using a both borrowers income to demonstrate they have enough money to pay the loan back. Even if someone makes enough to pay us back, it doesn't mean they always do - this is why we look at credit scores/history. The best way to determine how someone will meet their credit obligations in the future, is to look at how they have done it in the past. Since we need both people to use their income to pay us back, we need to evaluate both borrowers credit history. If one of the two borrowers doesn't contribute to paying the loan back, the risk of us not getting paid as agreed goes up. Therefore we evaluate risk bask on the person that is the most risky. There is the option to not put the lower credit scored person on the loan and qualify only with the other. If this person can't afford the home on their own, and depends on the others income then we want to know how they used their money to pay debt in the past.