...because of unexplained, pre-existing sulci and/or gyri formations which may be incompatible with implants. Since the brain is an organt that is continually growing and changing from birth til death, it is possible that implanting new cells that are not familiar with the "habits" of the old cells could result in damage. Such damage has already been documented as "tumors" by previous researchers (www.wired.com/medtech/health/news/2004/11/65735) and are known to act as obstacles to attainment of a viable neurological stem cell implant. I believe that overcoming this obstacle would be a matter of a systematic understanding of progressive growth of nerve cells with regard to their reaction to various stimuli both at the cellular level as well as in the brain as a whole. Knowing this could help us better understand how the brain would react to an implant and aid us in either creating the appropriate conditions for an implantation to take place, knowing when a subject is a viable candidate to receive an implant (i.e. the subject already possesses a brain in which the appropriate conditions exist) or knowing when implantation is not an option (i.e. appropriate conditions do not exist and can not be created artificially or otherwise)...
The research has probably already been done, though. I'm always late when it comes to these things. *shrug* Oh well...
If you know of any relevant works, feel free to post articles.