Board Thread:Wiki Discussions/@comment-24936031-20140513051946/@comment-3511763-20140513060207

The closeness in blood plays a part in success or failure. Since Madara and Obito are close in blood seemingly, the transplant failed and didn't result in a new EMS, but still allowed Madara the use of Kamui with just the MS. That, and it has been shoved down our throats that eyes work better when they are paired with their true genetic counterpart.