It would seem likely a brain stem lacking organism is an easier build than a single organ. All the cellular, organ specific organelle, organ and structural support is given, so the extra bits needed to feed and shape an organ are found organically so to speak.
In effect, editing out the brain may be simpler than building up a standalone kidney or liver. It's a sum of the parts greater than the whole thing. A standalone organ assumes the endocrine and other signalling is faked out along with nutrition and oxygenated blood. If you grow up a body from cells that divide and specialise you get all the mcguffin for free.
Plus, for a needed kidney, you get transplantable skin, liver, bone marrow, bones...
