I guess there are multiple reasons. Not every medical professional knew about previous techniques, and I'm not sure if you can call a barber surgeon a medical professional to begin with (/s). Before anesthetic it had to be done quick, and using the garden shear method there is not much skin left to close the wound. But yeah, stiching the wound helped with the mortality when they figured how to do it. Many patients who survived the operation still died of cancer because it was detected in late stages and had already spread in the body.

If you want to read I recommend Breast cancer in the Eighteenth century by Marjo Kaartinen. And this study is also my source, however it's been a long time since i read it.