Because boas, it has been proven, can not process all the extra fat in the rodents we feed like pythons can. Keepers have lost boas due to over feeding. Especially over feeding obese rats. In the wild boas are semi arboreal and their diet is varied. They eat many birds as well as mammals.

The rule most of us follow is to only feed our adult boas after they poo. Feeding a boa the same way you'd feed a ball python causes them to grow much too quickly, shortens lifespan greatly, causes fatty liver and other organ issues, problems reproducing, many slugs, small litters, etc. They do not need a huge meal that puts a big feed lump in them. Slow growing a boa is always best for them.